Been a few days, and I'm a bit better informed now.
The problem with Stoat is that it's UK-based. So, the nanosecond that gets popular it will get pozzed by that fake country's failed government, and we're back at square one.
I've been investigating Matrix, and it's a bit more involved than I thought. Matrix is actually the back end of an end-to-end encryption network. The neat part is that there's many different front ends, so if you don't like one client, you can swap to another.
Of course, you actually need people to talk to, so it's back to waiting for people to move. Of course, nobody will move when everyone is waiting for people to move, so it's the same fantastic tragedy of the commons bullshit as Youtube and every other big consolidated platform.