And that they do not delete your messages, except you delete everything one by one per hand (or script), which most people would never do. Oh and don’t forget the private send files that are fully published (you can open them via browser!). This is something many people do not know. The security was a tiny bit improved at some point, to make it harder to guess the file links, but they’re still published.
Signal is no good replacement for Discord. At least if you rely on features as screen sharing and multi-accounting (you need a new phone number for each account, as far as I am aware) and who knows what else. But Matrix is a good option, which is widely used in the FOSS world.
Anyway, the more important point is, that people don’t need to use Discord OR and alternative, they can use both until people joined the other platform, too.
Back to topic:
It really makes no difference if they collect some additional trackers-data, since all the important data can be read and analyzed even if you use another client (even Matrix-Discord-bridges). What you probably don’t want is a system access where Discord can read what games you start (and every other application). I don’t know if Flatpak has a strong enough sandbox to stop this, but that is something you can easily test by your own. If it still shows the games you’re playing (even if disabled in Discord, it should be visible in its settings), you may prefer a third party client. If it prevents Discord from doing so, I see not much reason to switch the client other than UI design etc.