Oh i agree on that, but matrix itself is open source, that's why i listed it too. The 3 options are good, but the more secure the more complicated.
Beeper is the easiest, with the con of closed-source client (the server is open source and they contribute a lot to the matrix ecosystem, thus my trust in them). This is what I'm currently recommening to most people as a lesser bad just because how easy it is to use.
Public matrix instances are good too, i personally use frei.chat, but most of them use unmodified matrix servers and there's a bunch of open source clients available. Bridges with other networks are configures through bots.
Then you can self-host your own server, but it can get messy and expensive. I usually prefer to self-host everything, but decided to pass on matrix for now.