What is the history of your A5 as the v4 was discontinued in 2020/21 ?
fwiw, if 5 GHz was misbehaving with stock firmware, have you considered the 5GHz radio in your A5 may be faulty?
I briefly owned the EU version of the C50 v4 many years ago. I don't recall any issue with 5 GHz radio. But there was a Poor Range issue with the 2.4 GHz radio at the time. It was fixed in 19.07.7 onwards.
https://forum.openwrt.org/t/archer-c50-v4-mac80211-loses-internet-access-after-20-away-from-router-but-maintains-connection/53575/4
The v3, v4 and v5 all share the same Mediatek chipset.
Try 19.07.7 ?
If problem persists, your A5 may be faulty....