Signal has been a good option because you can get "normal" people to use it, which hasn't been true for many of the alternatives (except Telegram, but that's a mess).

Signal used to be the best answer to this conundrum, since it would use its own internal protocols if it could or fall back to SMS if it couldn't, unfortunately they decided to drop SMS support a few years ago, citing users that sent sensitive information not realizing they were using SMS (that always felt kinda flimsy). I really disliked this change, because it raised the difficulty of adoption, from just getting people to replace their default app with Signal to making them manage multiple apps.

Now though, you basically need to advocate socially for the change you want to see in the world. Anecdotally, I started using Signal when they still supported SMS to talk with 1 friend group, and eventually convinced most of my closest family groups to also use it, many after SMS support was dropped. Apart from 1 tech illiterate elderly couple and 1 extended family member, I haven't received any personal (non-company related) text messages in like 5 months.

If you quickly uninstall it because you don’t know anyone using it it sounds like you’re part of the problem. If someone you know installs it to try it out that’s one less person they see as well. Personally I got the vast majority of my friend group to move to it years ago by just saying like “hey Facebook sucks we should move to signal”. If you don’t want to do that should at least leave it installed it’s not like it’s taking up much space

You can download the APK once from the website, after that it updates automatically. https://signal.org/android/apk/

I use obtainium to install it and it works flawlessly. The reason they don't publish it elsewhere is due to licences and push notifications iirc