One last thing though: despite having added “hdmi_enable_4kp60=1“ to my config.txt, I am still limited to 30 fps in 4k resolution. I had the problem before and I still don't know how to fix this. Maybe I should create another thread?
Yes, but use our forum search first. We already had that 4kp60 topic several times.
Thanks for testing! Do you use a DVB-T tuner on USB? If yes, then you probably know about the case of harry_fine , which appears at your first link. On that thread we spoke about nearly every potential bottleneck. I don't think I can add new ideas to that topic. I'm just waiting for feedback from harry_fine ...
Read this thread: Click!
PS: If you are using custom settings at config.txt, go back to default.
Nice 
! You could convert problematic movies into another format and / or wait until the issue has been fixed on LE.
Yeah, that error is visible at the log:
2020-12-30 20:04:22.687 T:3011665936 NOTICE: Whitelist search for: width: 1920, height: 1080, fps: 29.970, 3D: false
2020-12-30 20:04:22.689 T:3011665936 NOTICE: Display resolution ADJUST : 3840x2160 @ 30.000000 Hz (16) (wei-ght: 0.000)
2020-12-30 20:11:42.195 T:2394477424 NOTICE: CVideoPlayerAudio::Process - stream stalled
2020-12-30 20:11:46.789 T:2989454192 WARNING: ActiveAE - large audio sync error: -1040.734258
2020-12-30 20:11:46.839 T:2989454192 WARNING: ActiveAE - large audio sync error: -1038.823835
2020-12-30 20:11:46.889 T:2989454192 WARNING: ActiveAE - large audio sync error: -1036.845208
2020-12-30 20:11:46.939 T:2989454192 WARNING: ActiveAE - large audio sync error: -1036.899898
2020-12-30 20:11:46.990 T:2989454192 WARNING: ActiveAE - large audio sync error: -1034.976967
2020-12-30 20:11:47.040 T:2989454192 WARNING: ActiveAE - large audio sync error: -1033.747706
2020-12-30 20:11:47.090 T:2989454192 WARNING: ActiveAE - large audio sync error: -90476.128866
2020-12-30 20:28:06.076 T:2394477424 NOTICE: CVideoPlayerAudio::Process - stream stalled
2020-12-30 20:28:06.238 T:2989454192 WARNING: ActiveAE - large audio sync error: -4911932.189470
2020-12-30 20:28:24.990 T:2419655536 WARNING: CRenderManager::WaitForBuffer - timeout waiting for bufferSwitch de-interlacing off. Read here how to do it: Click!
Try that setting with and without sync-to-display.
Which Raspberry Pi hardware?
Not sure whether it's possible on LE. I read about many weird audio fixes for RPi4B, and I'm still thinking that internal data bus is the cause. Sometimes different cache settings did the trick, sometimes switching off de-interlacing did it.
I think USB data rate is too high to stay in-sync.
There is no quick solution for this. You could wait until LE is efficient enough, or you could buy a HiFiBerry DAC. Because HiFiBerry DAC uses GPIO instead of USB, it can handle high audio data rates more efficient.
As I learned recently, the Kodi process starts after autostart.sh execution, so this approach should work for you.
I just received some feedback. Bluetooth settings need an overhaul. So you will not get a fix for this very soon - sorry.
I'm not sure whether this issue has been fixed (new hardware). I've been informed the dev - stay tuned.
LE installs the right firmware at installation process. It's possible to update the firmware after this (using Raspbian is the easiest way), but as mentioned above, don't do this without a good reason.
Make sure you are using a blue USB port and try one of those test builds.
It doesn't make sense to add questions to a solved thread. All users with open questions should open a new thread.
The new Wiki is work-in-progress. Please continue to give us hints, which topics are missing. Thanks!
This hardware is quite unusual. Most gamers use USB controllers. If you post the links to the scripts, we can hopefully give some hints.
You could activate log level 2 at advancedsettings.xml and read kodi.log after boot.