Seems you edited your comment to add new points. Lemmy doesn't notify users about changes to comments/posts they replied to, unfortunately.

Phone numbers required for now. Usernames potentially on the horizon per sms removal update.

Good to know. I prefer to never use my phone number for sign ups.

Server side code is open source and available at Signal Server Github.

They seem to put a lot of effort to centralize their app though - you can't setup your own server and talk to other signal users.

Android has an apk download so you dont need Google Play.

Yes, but you need to figure out how to verify signature keys on android + manual updates, it's just inconvinent if you compare it to F-Droid Signal devs so dislike for no good reason.