Matrix is also decentralized/federated, has encryption integrated into the protocol and enjoys a broad adoption and public support. It also has pretty good integration of bots and even other message protocol services like IRC via "bridges". The chat clients are pretty good too; Element is pretty much available for every platform but there's other one's which are more focussed on Desktop or mobile usage, depending on how you primarily use it.

The main thing they lack is an easy to use Discord to [XMPP, simplex, session] bridge, If they have that then you could just hop on whichever you prefer and be in touch with the group. Love it or hate it, discord is the top chat app for the moment and being able to interact with it is a necessity.

Matrix has basically come in to place as IRC "2.0" with developers, nearly every fediverse project has Matrix channels, most open source projects in general are starting to have them. XMPP has many years on Matrix but it never took off in regards to that very critical niche of early adopters, developers and power users. XMPP was a good competetor to the web 1.0 chat apps of AIM and MSN but Discord and other apps are much more rich content focused.

Simplex seems to only have a command line interface for PC, it als owould be pretty clunky for groups i think, I have yet to see anyone advertise a Simplex "group chat" in the way they would a discord server, but perhaps there are examples out there.

Session is somewhat better but hinges on the success of a cryptocurrency for its continued development so thats a no from me, also seems to have a lot of scammers although perhaps not as bad as telegram.

Matrix has clients listed on F-Droid.