Updated to the latest beta, all pretty smooth! I have just a two extra observations and comments.
-I metioned about not being able to play a sample 4K HDR video before, (the Sony sword-making one). I've tried played it from a USB stick directyl connected to the Pi and playback was much smoother! Turns out my NAS is simply too slow to keep up. The video is 60fps, really high data rate I guess.
-Using the build-in File Manager once I access enough folders, I cannot return to root... Is this a common issue? I'm accessing my NAS server.