thanks for trying the current Mikritik OpenWRT Firmware, I just wanted to try that myself this evening, you saved me a bit of time there:-)
I also bought a Mikrotik OmniTikG-5HacD (OmniTIK 5 ac) for my Patio and thought, for Accesspoint Only I get along with RouterOS.... but neeeeeigh....
I am currently not able to convince the switch (why is there a switch in the first place on a Accesspoint type device???) to pass through vlans to the appropriate ssid, despite trying three different approaches of which individually should work on routeros.... but even with a traffic dump on the device itself, I see hat the traffic reaches the device but is not forwarded to (through) the bridge to the wifi ssid at all....
This device is very similar to the hap ac, which was supported by ar71xx
Some time ago I built a quick test ath79 qca8k based image for both of these devices. You should be able to get some hints from it at least: omni ac
It worked for me at the time, and given my device is now functioning, and out-of-reach, I have been reluctant to change it.
Not submitted to Openwrt:
master ath79/mikrotik does not use qca8k
kernel 5.4 qca8k does not support switch vlan offloading, and the backport is not straightforwards
on the hap ac I could not get the SFP port functioning correctly (needed fixed-link, and link reset).
voltage / temp sensor support not included (kernel driver works, but needs hacky DTS)