Question is in the title basically, I got an EPIC .mpr that I want to convert to .mp4 for uploading in YouTube and similar, but it's long and I don't want to screen record the whole thing.

No.

You must screen record.

An .mpr is just a record of the positions and other game state. The game engine needs to replay that data to be able to create a movie of it.

Unfortunly, it isn't quake, so such function does not exist. And guess getting libav injected into the game is unsound idea for the devs, but who knows, maybe one day they'll get some freak onboard who will think of this.

There is LFS Record: https://www.lfs.net/forum/thread/72001