I agree, I personally really dislike discord - I do get that it works better for certain group sizes, but still.

Given that most of the clients are open-source, I wonder why signal or someone else didn't just change the "backend"

I really don't get the appeal. I mainly use Signal, though I also use Telegram a lot, and Telegram seems a lot more dodgy. I constantly get spam on Telegram even though I never get spam on Signal.