ffmpeg / libav had lots of bad blood long before the fork, which might have been the real driver for making it a "debacle".
GitHub in particular has a "fork" button to make it super-easy. So fork, do your edits. If you want, make a pull request with the original project. If they take it, great: the fork can cease to exist.
If they don't want to merge it don't take it personally but go your own merry ways. And this is what didn't happen with actix.
Forking if it's mature and featured enough to justify it, but it maybe too unsafe broken to invest in rewriting large swaths of code. That's the perpetual trade-off of engineering decisions; whether to build, fork or buy.