Signal sucks, a lot. Try using their "privacy first" service with a VPN. You literally need so solve a captcha on every new chat you start.

There is no official FOSS client. You can use Twinhelix' Signal-FOSS which works great. I recommend using Molly though, which has more needed features like linking multiple mobile devices (for example using an Android Tablet and a phone, with the full version instead of the uncapable Desktop version).

Signal is still based on phone numbers, which is its biggest strength. But it is a centralized Service, nobody runs a Signal server (apart of the country servers from Signal themselves).

Try SimpleX. It is really great and now has like eveything you need. It is completely anonymous, but can also be different. It gives the users a lot of control, which may not be wanted and confuse people.