The Telegram clients are actually open source and I think pretty solid. The Matrix protocol is open, federated, and secure, but the clients are kinda janky. I've been playing with the idea of what if someone were to fork the Telegram clients and modify them to work with Matrix. That would be a "best of both worlds" situation I think.
Try DeltaChat: https://delta.chat/
From a product point of view, maybe that's a fork of Signal? Maybe Signal Group, or something along those lines?