Nafeon

I use smartgit it serves me well: https://www.syntevo.com/smartgit/

And it has all 3 of your criteria as far as I can see.

“It magically appears in flatpak only after you’ve installed it by other means”

I kid you not this happened to me 3 times already.

I’m one of the guys on the couch.

The megalodon author themselve recommended to use moshidon and is considering becoming a contributor there.