I would not consider the AX53U a capable router for 300/20 Mbps ISP service with SQM.
If you use fq_codel/simple SQM and install the irqbalance package, the dual core / quad thread MT7621 CPU will struggle to handle ~200 of your 300 Mbps service. With CAKE, that will drop down to ~100 Mbps, but since CAKE runs on a single thread, it will at least leave some CPU to spare for WiFi.
Can you find a Belkin RT3200/Linksys E8450? This will give you enough CPU for 300 Mbps SQM.
Thanks for the information. Unfortunately neither of those routers are available here. I'll have to browse the assortments and compatibility table again. I feel a bit like I'll end up having to buy a device outside Finland.
Another option (most would say a far better option for faster ISP service) is to have a separate router only device, and add an all-in-one router/WiFi box.
For example, you could purchase a NanoPi direct from FriendlyElec to use as your router for SQM, VPN, etc. and combine it with an AX53U to provide a dumb WiFI AP and extra switch ports to plug in hardwired devices. Or you could combine a NanoPi with a dedicated managed switch and one or more dedicated AP's. Lots of options.
The NanoPi R4S (4GB version without EMMC) is supported by OpenWrt. If you decide to get it, get the high quality machined metal case it is offered with for little extra cost. The R4S will handle Gbps SQM no problem with plenty of CPU left over for other tasks. The R2S cost less and will also handle SQM for your current 300/20 service. However, for just a little more cost, the R4S will be far more future proof and have more CPU left over for VPN and any other tasks you do not yet know you "need."
Another option for a router only device can be a new or used low power x86 thin client to which you add a network interface card. Unless you want to route >1 Gbps, I think the NanoPi R4S is a far better option. The R4S will probably use less power (it idles at ~2W and max is ~7W), take up less space and cost less.
I managed to bring up the Bridge Interface tab in FritzOS. However, I don't really understand the logic of it... if the purpose is to disable the router, or set data traffic to bypass the router, why can I only set ports LAN2, 3 and 4 to bridge mode? Or does editing the exported file change the LAN1 port to a WAN port, which also bypasses the router?
NanoPi sounds insteresting. I watched some videos on how it works and how to configure it, it seems quite cumbersome, but I guess I can learn to configure it.
I only have two devices connected to the network: a PC and a TV. I've never needed wifi. The NanoPi only has two ports so should I connect my PC to NanoPi and my TV to Fritzbox LAN2 or 3 or 4? Or should I connect the NanoPi to Fritzbox via one of the bridged port, not to LAN1?
Then there is the micro-SD card thing.... Do I have to have a card reader on the PC to install the stuff on it, which then installs OpenWrt and LuCI on the NanoPi?
If I decide to buy a NanoPi, do I choose "4 gig RAM (no unique mac)" + "Combo with case" + "5V 4A adapter" + "Plug adaptor"?
That likely will be AVMs secret forever... AVM decided that they want to market/sell on their value-adding components and stopped offering settings to turn a FritzBox into bridged-mode, sa in that mode the FritzBox essentially acts as glorified media-converter (OK, for DOCSIS it seems to require a tad more, but AVM also removed bridge-mode for DSL long ago). Now with this option not being part of the official feature set it will likely neither be tested nor adjusted when default port mapping changes. So I am not sure whether it is all that worthwhile to ponder the "details" of the bridge mode, no?
I honestly do not know, last/only time I used docsis (3.0) was a decade ago and there the ISP only delivered a bridged-modem by default, getting a modem-router would have cost extra (fine for me, I wanted to use openwrt/cerowrt anyways), so I guess you should take my advise here with a grain of salt.
Looking at the 4RS's specification, yes it has 2 ports, allowing a conventional split with a single WAN port (to be connected to the Fritzbox) and a single LAN port (so you would need an additional switch to connect both devices). But e.g. the nanoPi R5S seems to offer two lan ports (the quad A55 is IMHO weaker in performance than the 2 A72+ 2 A53 in the R4S, but not by all that much). However the R5S seems not yet supported by OpenWrt so you would need to use friendlyelectrics OpenWrt derived OS.
I think you should get an SD-card reader/writer for your PC if you go the R4/5S route, but these should be pretty cheap nowadays. Not sure whether the EMMC versions do not already offer the OS on EMMC, in which case the sd card writer becomes optional, but might still be a good idea, if only as back-up.
The R5S is definitely fast enough for your current ISP service (~50% faster than the R2S, due to its higher clock rate more than its A55 versus A53 cores in the R2S), but I would still get the R4S, because the R4S is ~50% faster than an R5S, and the R5S is not supported by OpenWrt. FriendlyWrt is reported to have security issues.
Yes.
FriendlyElec's prices are quite reasonable for the power adapter and cable they offer, but I can tell you from experience that the 20W adapter from Friendly Elec is a bit large to play nice with another device plugged into an adjacent power strip plug. If that could be an issue for you, you may want to shop around for a smaller power adapter as long as you do not plan to plug things into both USB3 ports that need power.
For the R4S, figure on ~5W per USB3 port (it has two) and ~7W for the CPU - so it makes sense that FriendlyElec provides a 20W power adapter. But since I plug nothing into the USB3 ports on mine, I dropped back to a smaller 15W power adapter.
Maybe it's best to give up on the fritzbox and buy a modem with easy bridging or a modem without a router.
So the FritzOS bridge mode will not turn the modem into a pure modem, but will also offer a bridge in addition to its router mode. As long as the bridged LAN ports work ad expected with your ISP why exchange the fritzbox? You should have no need to access FritzOS regularly, so you might simply unplug from the bridged port and plug into the non-bridged port if you ever need to change something in FritzOS?
Now you can try to buy a pure bridged modem, assuming your ISP will provision it, as not all cable ISP will provision any arbitrary modem. But since you already own the fritzbox, why not simply use it?
The Fritzbox 6591 uses an Intel Puma 7 SoC. The Puma 6 and Puma 7 both have latency issues due to defects at the hardware level. Intel issued some firmware updates to mitigate the issue, but it was not fully resolved. The general consensus seems to be the Puma 6 still has problems no matter what, but if a Puma 7 device can be put into bridge mode to take routing load off the CPU, its performance as a modem only device is reported to be acceptable.
If you can put the Fritzbox in bridge mode, you have nothing to lose by trying before you buy your own ISP compatible modem.
It also depends on how inexpensive used modems are on ebay in your area. I was able to pick up a used 24x8 Motorola MB7621 DOCSIS 3.0 on ebay for so little it was essentially free. That may not be the case where you live.
I made a few ping tests. Connected via LAN1: google.com = 39-45 ms. Elisa.fi: 15-21 ms. Connected via LAN2 (bridged): google.com = 11-14 ms. Elisa.fi 8-12 ms. The difference is due to the Puma 7 issues?
Well I guess I start with FB/NanoPi combo and consider bying a new modem later if necessary.
Not knowing any of the details, I would recommend to perform such tests over longer time, e.g.:
mtr -ezb4w -c 1000 google.com
to collect the RTT for all hops from your computer to google for 1000 samples (this takes ~ 20 minutes, so reduce the 1000 if you are more impatient). MTR (or Win MTR) reports minimum, average, maximum, and more which over 1000 samples gives a better picturre of the achievable RTT.
Another option for a router only device can be a new or used low power x86 thin client to which you add a network interface card. Unless you want to route >1 Gbps, I think the NanoPi R4S is a far better option. The R4S will probably use less power (it idles at ~2W and max is ~7W), take up less space and cost less.
The R4S with case is $75 from FriendlyElec. Used Wyse 5070 thin clients, with Celeron J4105 CPUs, are readily available for less than that. I'm not sure about the power draw - the specs say 4W idle, 7W running, but they don't have router usage in mind 
With power supply and some storage included in price? And before, or after adding a network interface card for a second port? Regardless, those do look like a nice option if they will accept a NIC card. A bit more CPU on them than an R4S too.
Keep in mind that you either need to use a managed switch (for a router-on-a-stick configuration using VLANs to separate WAN and LAN traffic over the same singular ethernet port) or you need to get an additional ethernet port (e.g. USB3 gigabit ethernet dongle like tp-link's ue-300 to name one with decent reputation around here) or find a way to use the M.2 slot to attach ethernet. Both options will work, but neither comes for free, potentially changing the value comparison with the R4S somewhat.
There's this listing, for example, which includes power supply and 16GB eMMC for $55 shipped. A second NIC will need to be added, though. And the R4S price of $75 is without power supply
Understood, but a USB3 adapter costs about $15, and with the 5070 available for $55 shipped (in the US), it seems to compare quite favorably on price and performance, and at least come close on power usage, to the R4S.
Is there any requirements for the SD cards on R4S? Minimum/recommended size and such?
What about writing the needed files on the SD card? Any requirements for that?
And lastly, what format method is the correct one... NTFS, FAT32, exFAT...?
i use ext4 on mine
The R4S will accept up to 128 GB SD card. I use a 32 GB card on mine, because that capacity is very inexpensive and I do not need more than MB's for my use as a router anyway.
Formatting your card as ext4 is far more flexible if you plan to make full use of the R4S with docker, etc. and want to utilize all the SD card storage. You can also enlarge the ext4 OpenWrt partition to make room for a LOT of packages. Disadvantages of ext4 include: a) every time you upgrade OpenWrt, you will need to manually set up your card partitions again, and b) if you need to reset the R4S, you will lose all your installed OpenWrt packages and have to reinstall them, and OpenWrt if not running a stable release, again.
An advantage of formatting your sd card as squashfs, provided you build your image using the Firmware Selector, is that if you reset the R4S to defaults all your packages that you select to be included with your Firmware Selector image will be retained. The disadvantage of squashfs is that it does not leave you a lot of room to install lots of package and it makes no use of the rest of your SD card.
I use squashfs, because I use the R4S as a basic gateway router and nothing else. I do install a fair number of packages and I still have 86 MB left for more packages. It meets my needs with margin to spare. But I do lay awake at night worrying about all those GB of storage on my SD card going to waste 
R4S is now powered and connected, and micro SD card is formatted (ext4) and the firmware is flashed.
The setup guide is more cryptic than I expected. Is there any guide for beginners? I followed the quick start guide here https://openwrt.org/docs/guide-quick-start/factory_installation
Section "Flash the firmware"
What went wrong?