As for OBS and MKV/MP4. there's a feature called "Remux" where you can convert MKV to MP4. I use it all the time. super fast. You can find it under "File/Remux Recordings"
But yeah, the video feature in Godot is great.
Currently we don't support direct uploaded videos on the forum I'm afraid. This is however something that's being looked at. So for now you would need to embed the video from another source (like YouTube, Vimeo etc. ).