After being through hell of testing the last two days, I may share my gained wisdom
Running the monitor at anything above 1080p, like 1440p (2560x1440) or 2160p (3840x2160), OBS starts to stutter and I just cannot manage to find a good quality recording.
Relive for the rescue!
Relive records seamless, no matter what.
I only went for OBS, because a while ago Relive audio was often not in sync, they seem to have fixed it.
1440p and recording to 1080p with Relive, no matter what the bitrate is, the sound sometimes become roboter like. I mean it sounds like you are talking over a radio.
2160p and recording to 1080p with Relive, no matter what the bitrate is, the sound is always fine.
I did not notice much difference between AVC (AMD264) and HEVC (AMD265), but will record in HEVC because it supports larger resolutions.
Forget HDR if recording with Relive, the colours are awful and all over the place.
Recording size is quite large, you get a 2min video at ~200MB.
Youtube will convert the H265 format straight away and fast, here some test (you can see the MSI Afterburner fps top left, with Freesync no problem):