I hope sliding-sync is merged soon, so it is usable enough that I can invite my friends to our matrix server

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.

I have gotten my friend group to switch to using Signal for our group chat. It was pretty hard because I'm the only Android user but I ended up convincing them

The culture war is v stupid, yes. But "use signal" is the same as "dont text your friend with a different app at all" with signal dropping sms. They are in a different position that google and apple, so not saying i blame them for dropping, but i did stop using it once it become "another app" instead of how i text.