Redmi is a more economical sub brand of Xiaomi and both usually sell very similar hardware.
Redmi usually remove things to bring the price down, heat sinks, switch leds, etc. but essentially they are the same device.
ATM I don't think Xiaomi or Redmi commercialise any of these routers in Europe.
I know what Redmi is, I have been using Xiaomi phones since Mi3.
They dont sell any of the new AX routers, but they have announced to start selling them in Europe in July
Asus created something like this RT-AX56U
but that's not all
on firmware site I saw SOURCE CODE (select a different or other operating system)
maybe it will be helpfull !!
As far as I know, the problem here is the missing Linux kernel module of the broadcom chip.
Its missing drivers for the whole SOC family
Given that this thread started 2 years ago, is it fair to say we are still no closer to a WiFi 6 router in OpenWrt because of drivers? It's sad but just curious, my WRT32X OpenWrt 19.07.3 on kernel 5.4.42 has been running amazing but always looking forward to the next thing.
As usual, Broadcom has little to no upstream stuff.
So far only Qualcomms ath11k and Intels iwlwifi are upstream AX drivers, but Intel does not support AP mode.
So essentially only Qualcomms IPQ807x and IPQ60xx series are worthy of porting to OpenWrt as Qualcomm has put some effort to mainline them, but a large portion of drivers is still missing
Mediatek mt7915e support will follow in the future as well, it's on its way upstream (part of kernel v5.8~ already) - and the current trend of vendors combining mt7915 with mt7621a will allow for easy porting, at the expense of being stuck to a rather slow SOC (which probably isn't quite up to the task of an 802.11ax router).
Personally I'd prefer ipq807x as well…
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…also considering the current state of mt76 for 802.11ac, namely mt7602e/ mt7603e/ mt7612e/ mt7615.
mt7915e is already supported by the mt76 version that is in OpenWrt because OpenWrt uses mt76 directly rather than using the in-kernel version.
Yeah, it's probably gonna be an easy port but that CPU is not gonna be up to the task, especially since there are no offloading cores like NSS/NPU ones that QCA has.
I would personally rather buy IPQ60xx based boards like Xiaomi/Redmi ones, its gonna take time to get them working properly but the CPU is much more powerful.
And the price on them is really low even now
Overclock 
That's not going to do much. Max overclock is 20 MHz for a total of 900MHz.
That's not very smart to say, because wifi 6 has Dual Carrier Modulation AKA DCM built-in, which basically extends the range by 25% compared to WiFi 5 or WiFi 4, which is very significant even for single user usage in household environments.
Plus the bss coloring and 2.4 range are GREAT improvements for people who live in apartment buildings
OFDMA is a central feature of 802.11ax, but at this point it doesn't really work:
Redmi AX5 43€ how is the device and the price?
It seems to be a clone of the Xiaomi ax1800
https://mx.gearbest.com/blog/new-gear/xiaomi-mi-router-ax1800-unboxing-and-teardown-12503
So it's bad since is should have low ram and space
Well, its designed to be as cheap as possible.
BTW 256MB of RAM is minimum supported by QCA reference designs, there are some interesting methods they use to reduce RAM usage.
nss makefile are full of this measures
Yep, DTS files for IPQ60xx series also.
They even have memory.dtsi that is full of ifdefs about memory size so reserved memory for a lot of services shrinks severely with 512 and 256MB of RAM.
Heck, even ath11k has different properties based on RAM size.
upstream reviews will be very funny with all this extra code for small memory
Not actually, they have gotten smarter so they dont even try to push stuff like that.
They will only push the minimum of reserved memory nodes and its up to you to add the rest to your board DTS.
If you though that IPQ8064 RPM regulator stuff was complicated, then take a look at IPQ6018.
It all obfuscated through various mailboxes, secure FW calls etc.
I mean, they dont even have voltage and frequency scaling upstream.
Heck, you can even run the CPU at 1GHz+ as only couple of days ago patches to enable the PLL supplying those clocks has appeared.
I tried to figure out how to configure the RPM regulators myself, but not luck as downstream DT bindings differ completely.
So I am stuck waiting for Qualcomm to start pushing DTS changes for the new drivers themselves.
I dont even want to go to networking side, that is another mess.
ath11k will even fail to ioremap into the designated memory space, drivers for the ethernet are NSS-DP that is tangled into SSDK and none of those work after my porting.