The only thing worth mentioning here is that HD audio won’t happen until we do a Debian Bullseye (11) release as we need a newer version of libasound. The current Debian Buster (10) version is too old.
I suspect that’s a bit over a year away.
MMAL is still used / supported on these devices, but the V4L2 interface is the direct API used via Kodi. V4L2 is a shim around MMAL.