Signal FOSS is a fork that takes out the closed source bits. More info here:

https://www.twinhelix.com/apps/signal-foss/

It still connects to Signal's centralized server and doesn't provide an option to connect to any other server. And that server requires you provide your phone number, but it's at least slightly better than thr stock version.

There are at least two other forks, but I can't remember what they are off the top of my head. None of them get away from phone numbers or centralization.