probably one of the best free video editing programs out there atm is davinci ( https://www.blackmagicdesign.com/products/davinciresolve/ )
for simple editing though I used to use KDEnlive a lot, pretty easy to wrap your head around, allbeit with a few quirks that take some getting used to ( https://kdenlive.org/en/ )
I'm sure theres others but those are the two I've used personally.
Yea, Mlem will get there I'm sure. Going off the fact Jerboa is on build 33 (0.33) I imagine its gotten a decent amount of stuff ironed out over the last 6ish months, Mlem is only on build 6 (0.0.6) so its still fairly rough. It works, but its got a bit before it is better than just using the mobile site through browser.