there is matrix, the best option, both secure and reasonably well made(element and alternative clients), but it miles behind telegram in terms of UI/UX even though it's on par with whatsapp/signal/etc
The grandparent said mainstream and Matrix is far away from that. You _might_ count Signal, but its market share is still way behind and in an area with so many network effects, this is everything.
Is it? I just tried Telegram again today and something as basic as joining a public channel from the app itself (as opposed to clicking on a link) was seemingly unavailable, or at least hidden well enough that I was unable to find it.