Poor/unstable MT7615E performance on Cudy WR2100

Wifi 5GHz has poor / unstable performance on Cudy WR2100 with VHT-NSS 1 clients (no problems with VHT-NSS 2) running openwrt 21.02.0, snapshot (r17750-213ce1d837) and a 21.02.0 version provided by Cudy. The problem is not constant though, sometimes after giving the 'wifi' command a few times or rebooting the router performance is good...

With 21.02.0 and snapshot the VHT-MCS rates are constantly changing.(more details)

More people have reported this issue on various platforms, however it seems that this issue is not happening on all routers. Could it be something related to the Cudy WR2100 platform? Antenna settings, eeprom?

Does anyone have advice on how to collect relevant data to pinpoint the cause?

(I'm using stock firmware at the moment, but it offers very little apart from stable 5Ghz performance, so I would like to go back to OpenWRT...)