New AMD/X570 build - video & iMessage issues

Some advice sought from the hive mind here: I've just about completed my first Opencore build (Opencore 0.6.0) build with:

Asrock X570 Taichi/latest BIOS (3.2 IIRC)
3900X
2xSabrent PCIe 4 1TB SSDs
64GB Patriot Viper Steel DDR4@3200MHz
Sapphire Radeon VII
Corsair RM850X PSU
Corsair 570X RGB Case

Mostly working very well, but I have:

• persistent stutter, audio sync and freezing issues on video playback/display - everything from iPlayer to Logitech HD Webcam on Zoom. Videos play back in Photos just fine.
• video stops/breaks up completely when I launch another app
• the common sleep & iMessage/Facetime authentication issues - trying to get to the bottom of those

Any suggestions from similar experience? BTW, I'm not sure if the X570 Taichi has an option anywhere to switch off serial port (presumably the PS/2 port) - nothing visible in the UEFI.

thanks!