I think I should give up using RPi 4B to do the test with MT7612U, even with a proposed echo 1 > /sys/module/mt76_usb/parameters/disable_usb_sg fix (Scatter-Gather issue known for some not so good USB3 controllers) it won't work on USB3 port.
However now it works on USB2 but what's the point here? The speed drops by at least 50%, NanoPi R4S surely does the job much better here.
Moved on to test using my new PIX-LINK LV-UAC04 on NanoPi R4S.
This is the single state WiFi, can be used without USB mode switch.
Ran a few tests as AP, I can get up to 520Mbps download + 430Mbps upload, which is not bad at all, but since this one has no ext. antenna like the COMFAST one, signal level is obviously lower (same test setup, COMFAST can show roughly -23dBm signal while PIX-LINK is showing -40dBm)
One interesting fact was found during test: This dongle is a bit flat & fat in shape, so I tested with angled USB3 adaptor and the dongle kept throwing error, but when I direct plug to R4S USB3 port everything works!
Edit:
After a few months, I do have some updates regarding my 2 COMFAST MT7612U based USB WiFi (CF-WU782ACv2 & CF-WU785AC):
Previous posts mentioned that both of them are multi-state devices which need to use usb-modeswitch package to make them working, however I figured out that after reboot it won't come back automatically (I have no idea why, usb-modeswitch on normal desktop Linux works again after reboot but not on OpenWrt), without physical unplug/plug, or sending USB reset command with a script but it might still not working properly (at least a physical unplug/plug would render my previously configured AP not working).
After some searches, I came across the following GitHub comment talking about similar thing regarding COMFAST CF-962AC:
From there I learned that we can do a hardware mod by removing 3 resistors to change it back to single state! As my 2 USB WiFi also coming from COMFAST I guess they should be in similar layout, which was confirmed after opening up the cases, I ended up removing the 3 resistors and now both devices are single state now, no more issue with reboot on OpenWrt now.