I was thinking a webapp of an open source app such as Element (matrix) is a good candidate. Also converse.js, and Movim (xmpp).
Me being Swedish I totally expect to have one Android phone for my eID app, and one with an open source OS for everything else.
It was good while it lasted, making orders on my phone from anywhere in the world. But now I'll have to go home to get my eID device, which is fine.
This is obviously merely a short term solution since it should be clear that targeting hosts and developers of non complying solutions would likely be the next step.
There are very few relevant messengers and adoption rate is king, because at the end of the day we want to use these messenger to communicate with people.
Getting people to try out new/different messengers is already a pain. And here every blow against an uncompromised messenger to make them harder to get will lower its usercount and push more people towards the compromised software they can easily get on the store.
Sure I'd say some way of circumventing this will probably remain available for a while, but I'd say it is extremely easy to make this very inconvenient.
After all this isn't about having some way to communicate safely, this is about being able to communicate safely in our daily lives is what I'd say.