Actions are bad, but they're free (to start) and just good enough that they're useful to set up something quick and dirty, and tempt you to try and scale it for a little while.
Exactly. Any github alternative needs to consume same GithubActions syntax OOTB I'm afraid.
Which, as far as I found so far, means Forgejo. Haven't found any others. And even Forgejo Actions says that it's mostly the same as Github Actions syntax, meaning you still have to double-check that everything still works the same. It probably will, but if you don't know what the corner cases are then you have to double-check everything. Still, it's probably the best migration option if you rely on GHA.