Keep an eye on Whitenoise. It's basically taken the technology behind Signal and placed it atop Nostr, so rather than signing up with a phone number, you do it with an npub (pubkey). Still in very early days so the features aren't all there yet, and battery use could be better, but they've got the basics of it working already.
SimpleX is another option.
These don't have discoverability for lay people users just joining though, which is actually a huge network effect positive for Signal in the family and friends use cases.
However it avoids the issues with the public group chat privacy. It ends up coming down to client and protocol features for those. SimpleX has a more extreme privacy threat model than Whitenoise so user contacts tend to be throw away (for good or bad), which generally doesn't work for public communities.
The real kicker is that almost nothing has the community automation tools and administration of Discord which is the really hard lift.
I have lots of Signal contacts I cannot phone, since the phone number is never shared by default. Not even the signal contact is shareable. It is way too privacy focused to work easily.
i.e. I cannot even match two people I have in contacts unless one of them sends me their hidden username. Then they can talk to one another.
And people in my contacts don't use their full name. In groups, they often share the first name, making it confusing as hell. And many use an arbitrary nickname, most often the abbreviated first name I think but sometimes truly random stuff, and might even change that yearly with no mapping in my history to tell me who they were.
I, and all of my contacts, have the default setting for this which makes me discoverable on Signal by phone number look up, but I have phone number sharing disabled. That's the default settings.
I've had no issues at all with discovery.
Signal has had the ability to share a username instead of phone number for a while. You definitely want to pair that with not sharing your phone number with Signal contacts (the related option released at the same time).
Not a privacy app in my opinion. Sure, might be good for some use cases... but overall there are better solutions.