So it knows about all metadata

Metadata is encrypted on the client-side using Signal's sealed sender implementation. The client also removes as much metadata as possible. All of this is open-source and happens in the client application.

plus registration with phone number

Signal doesn't store phone numbers. It derives a user id from your phone number along with other parameters. It's in the open-source server code, you can check it out yourself.

you need to use the client built by Signal

No you don't. I myself use a fork of Signal called Molly.

with dependencies from Google Services and the like

Not true again. You don't need to use the official binary that includes Google libraries. These aren't required for the app to function. You can use Signal-FOSS or Molly-FOSS, and it works just fine.

and you can’t use one built from the source they provide

If this was true, forks like Signal-FOSS or Molly wouldn't exist.

Which at that point means they can introduce whatever they want in whichever version.

Stupid conclusion, because all of your previous points are false

Stop spreading false information, focus on the facts.