OpenWRT Mesh networking on pure 5Ghz

I am looking to setup OpenWRT based mesh networking but the 2.5Ghz band is very congested to it is out of the question. Would it be possible to do a pure 5Ghz mesh? I know some devices have multiple antennas but I am not sure what kinds of hardware supports both being a client and a server. Has anyone done this? If so, how did you do it and does it work?

Yes.

In an 802.11s mesh there is no client and server. All mesh nodes are peers. Almost all wireless drivers support 802.11s mesh interfaces these days, with Broadcom drivers being the obvious exception by not supporting it.

Please read the documentation:
https://openwrt.org/docs/guide-user/network/wifi/mesh/80211s
https://openwrt.org/docs/guide-user/network/wifi/mesh/mesh11sd
https://openwrt.org/docs/guide-user/network/wifi/mesh/rapiddeployment

Tell what is your country, say in EU you have only congested Ch36 non-wds channel for mesh

Typo: You mean non-dfs, I think...

You can use ch42 80MHz as it is non-dfs, but yes, not much choice.

Full listing of channels:

The best setup is 2.4GHz/40MHz AX (wifi6) for the mesh backhaul easily giving in excess of 1.2 Gb/s with beam-forming, but it is expensive hardware (although the prices are coming down).

42/36 overlap, it actually selects main signalling 20MHz automatically no matter what you select initially.

N00b here. Why 2.4 instead of 5Ghz or 6Ghz? Just anything that offers 1.2Gb/s would be good, right? The goal is simply to saturate the final connection?

My intuition said 6Ghz would likely be ideal in 2024 as it would free up the 2.4 and 5Ghz for all the clients.

Non-dfs 20MHz channels are 32, 36, 40, 44 and 48

Non-dfs 40MHz channels are 38 and 46

The only non-dfs 80MHz channel is 42

Ch42 completely overlaps channels 38 and 46 and is only valid for 80MHz

The channel allocations were originally set out in the ieee802.11 standard and are now internationally agreed, with increasingly diminishing local variations.

The chart in the Wikipedia link I posted shows it all clearly.

Just that what you do when you select 36 and main signaling band goes to 48....

It is not necessarily always best as it depends on what you want to achieve.
Remember the mesh is a backhaul to allow you to connect your access points together. So the backhaul radio needs better coverage than the access points, so 2.4GHz will be better from that point of view (it goes through walls easier etc etc in Noob terms).
But raw speed is not as good because the channel bandwidth is narrower (in general).
So it is a compromise with choices....

Ch 36 is only valid for 20MHz, as is 48.
If you choose 36 and 80MHz, you get 42 if it is working properly. I have seen some AC drivers get it wrong in the past, but seems to be fixed these days.

Must be a glitch in the matrix
image

No, your graph is correct. As I said:

So selecting Ch42/80MHz, it will show channels 32 to 48 in use, centred on 42.

Centered is numerology, the actual primary channel is often reported back as opposed to one selected initially.