Thanks for the power consumption comparison.
...this confused me, I'm not sure what you're saying 
I do want an OpenWrt solution its open-source transparency...
Are you're just saying that Mini PCs compare with more "serious" enterprise-grade solutions compared to "home" routers?
I got the NanoPi R4S because i'm hoping for fibre soon and my existing openwrt modded bt hub5 isnt powerful enough for that.
A review for it is here : NanoPi R4S SBC preview with OpenWrt and Ubuntu Core - CNX Software (cnx-software.com)
And the thread for OpenWRT on the r4s is here :
NanoPi R4S rk3399 4G is a great new OpenWrt device - For Developers - OpenWrt Forum
It is Snapshot or other builds for now. It is due to be supported in the new 20 build once its released.
I'm using the anaelorlinski/OpenWrt-NanoPi-R2S-R4S-Builds: OpenWRT Builds for NanoPi R2S & R4S from official Openwrt source code with minimal set of patches (github.com) build to tinker at present.
Its a case of "grunt" as it were. Most home routers are cheap and cheerful. Limited ram and cpu power. As you step up towards enterprise kit the memory and cpu power increase. Like going from a little motoscooter to a Ferrari 
For an example, my bt home hub 5 is only good really for up to 300mb/s i believe. Beyond that you really need more cpu power.
Yes, and even if they are enterprise-grade, you can use it without problem at home.
Companies that sell "home" consumer wireless router generally offer hardware and software trimmed in all areas (Router, Switch, and AP), so that they at least meet the minimum requirements and earn more money.
As a sidenote, those mini PCs sometimes are the same hardware used in enterprise-grade solutions.
For example let's look at the appliances from SimpleWan
https://www.cloudco.com/simplewan-routers/
Oh look, it's a Supermicro Intel-based rackmounted server with a mini itx motherboard and a pcie 4 port gigabit card.
And what's on it? those two smaller devices?
Let's look at them from a different angle
https://www.ebay.com/itm/265173339814?nma=true&si=tDHmdTW21gTxWntIg1Ndv%252BF%252BMNA%253D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557
eyyy, that's a PCEngines APU.
Or the young company Firewalla:
Right now i'm using a simple TP-Link 8 port switch.
Ideally i'd like to replace that with a Ubiquiti POE switch but they are going for insane prices 2nd hand on ebay and are out of stock at suppliers. That would enable VLANs but i dont really require any at present.
I have a ubiquiti WiFi point that i've used to replace the Hub5's pathetic wifi. (I have concrete walls and even the Ubiquiti has some issues punching through)
Just a quick question. If it is just your wifi causing you issues. Why not get a more powerful wifi and add it to your wired network? If your router is working ok but for the wifi, just adding a dedicated wifi and letting it handle your wifi and disable the routers onboard wifi will solve your issue. I've had to do this with my bt hub 5. Its wifi couldnt punch through my walls so i got a ubiquiti AC-Lite and its been rock solid. I do however have a rasberry pi4 that i use to host the uniquiti controller to manage it. (It runs in a docker container and is FAR cheaper than getting one of their Ubiquiti "keys" which really is just a little dedicated device)
Some of the TP-Links and Ubiquiti's can also be flashed to OpenWRT but i like the ubiquiti interface. However the ability to put OpenWRT did make me lean towards their kit.
My internet and ethernet lines come into a small (~40x40x90 cm) telecom box with an electrical outlet inset in the bottom that is built into a closet wall in our house. This same telecom box houses a modem, an over the air TV signal amplifier, a small POTS telephone line distribution card with a few ports; and all the home ethernet, coax and POTS telephone lines enter this small box. The box cover is flush with the wall with some added ventilation holes. Despite mounting what I can to the back wall and carefully routing lines, it's just a bit crowded in there
A Belkin RT3200 / Linksys E8450 is just too large to fit in the box. I suppose I could purchase a Belkin RT3200 and remove the PCB board and mount that to the back wall. That's a thought. It may be less expensive too.
In any case, I place a high premium on my wired router solution being very small, having 4+ ports so I don't need to stuff another device (a managed switch) into the telecom box, and low power draw so that it does not produce a lot of heat (the modem does enough of that). The ER-X is nearly perfect for this application, but I'm starting to look for a little more processing capability than its dual core 880 MHz MT7621 provides.
Space limitations are also why I'm not excited about the NanoPi R4S - it only has two ports, so I'd also have to fit a managed switch into the telecom box. The Banana Pi R64 already has the ports attached, so I would not need to add a managed switch for my VLANs with the Banana Pi R64.
@D65 The suggestion made by @elan to use a Belkin RT3200 / Linksys E8450 is a really good one I think. I'm embarrassed I did not think of it after suggesting a Banana PI R64. Wifi range of this device has been reported to not be the best, but it's probably adequate, and that does not matter for you anyway - you are looking for a wired router. You can just turn the radios off in OpenWrt, and who knows-someday you may want to turn them on.
Well, I'm not completely sure it's only Wifi, it's just the most obvious there. My current router is also old enough that future OpenWrt versions won't be available if I understand correctly.
I'll have to draw a "map" of what exactly I want my setup to look like in order to make a smart choice. I'm glad I asked here, because I had no idea of all the possibilities and potential problems.
If you have all your kit in a box... You ideally want a external WiFi solution unless you have good range from that box. Unless you are buying new routers with WiFi 6, 2nd hand enterprise kit is FAR better than any consumer router kit.
The R4S is powerful and able to handle fibre speeds. Yes you'd need a separate switch but you can go as cheap as 20 quid for a cheap 8 port switch or go up to enterprise level switches with full L3 management. It also means you can replace all your kit separately instead of having to find a "Do it all" router/wifi/lan ports that you may have to compromise on.
Thanks, I will consider everything suggested here. I thought I was ready to buy, but it looks like I need to plan a bit more with all the new info so I'll sleep on it for a while.
I hear you. Heres some articles that you might want to read and have a think on. The Ars one is older (2017) but alot of what he says is still relevent today and points out a fair bit that you will also have to consider.
What I’ve learned from nearly three years of enterprise Wi-Fi at home | Ars Technica
This one is from 2020 and explains about wifi ranges and limitations.
The Ars Technica semi-scientific guide to Wi-Fi Access Point placement | Ars Technica
Actually, I do use an external WiFi solution. I have wired ethernet back haul routed from dumb AP's on each floor of the house (a used EA8500 and a used EA6350v3 courtesy of ebay) to the telecom box and plugged into the ER-X now. I was thinking of replacing the ER-X with a BananaPi R64 and turning off the radios on the Banana Pi R64 (I would not buy an 802.11ac add-on card for the BananaPi R64, obviously, but it has 2.4GHz and Bluetooth 5.0 on board).
You make a good point though. The NanoPi R4S is worthy of its name - tiny. And a managed switch is inexpensive and not too large either. I'll have to give that some more thought. I could probably make that work.
Smart move. The Ars Technica articles are amazing reads and well worth the time to read and understand all the issues around making your home digital. You can NEVER beat wired but in some instances there just is not a way to wire a part of house (be it access or spouse issues). So a decent WiFi is important.
Some advice I do suggest for anyone doing wifi? Make separate 2.4g and 5g networks. While you can do combined 2.4/5g SSIDs you will find that roaming will usually end up using the 2.4g signal as you move further away. Having separate SSIDs will keep you on the 5g signal and it can always fall back to your 2.4g if you get out of range of the 5g.
As for the R4S, I have been looking for quite some time for new router and i did seriously consider the APU series routers but at 200+ they are NOT cheap. The R4S i think i paid 80 for with shipping from China off www.aliexpress.com as FriendlyArm didnt ship to UK from usa.
FriendlyElec Nanopi R4S Mini Portable Travel Router OpenWRT with Dual Gbps Ethernet Ports 4GB LPDDR4 Based in RK3399 Soc for IOT|Demo Board| - AliExpress I bought the 4gb version with the metal case. I wanted to future proof it and also potentially move my docker containers from my Pi to my router.
My main reasoning for getting it was after having a long think about having a house LAN. It came down to "what if i need more ports." Swapping out a switch vs replacing a router was a much easier option and also means you can even go utterly bonkers and throw in a 48port enterprise switch with all the whistles and bells you could EVER need and still not have to pay $$$$ for a enterprise router/switch combo. By keeping router, Lan and Wi-Fi separate I can update/upgrade each as needed. Also it means if one falls over and dies I can work around it till i get a replacement rather than have my entire setup dead if it a combo bit of kit. (and yes if router dies i have no net but at least wifi works 
)
oh and if you are going to pickup a managed switch? Stay the HELL away from Netgear. They have moved to subscription based service and you require one to enjoy your managed switch at home. They also backported it into newer firmwares for older switches and caused a god damn riot on the forums when people updated and got "forced" into paying. Until they fix that crap, they are on my banned list for hardware.
Thanks everyone, you've been very informative! It's my turn now to figure out exactly what would be the best setup and explore the suggested options.