Your arguments don’t really make sense in the context of you asking for Threema instead, even if you acknowledge that it isn’t optimal.

[Signal] already showed that they’d like to keep the ecosystem locked down by not allowing 3rd party clients

Neither does Threema, it is a closed eco system? And in fact there is Molly, a hardened Signal fork for Android

At some point they will need a way to pay for their datacenters, […] i can see the pestering for donations getting much worse in the future.

Threema is a paid app though. So you consider an app asking for donations worse than a paid one? I agree though that their financials aren’t the best, since they seem to be living off a loan, but thats even more an argument to donate.