As much as would like to believe all promises coming from corporate execs - Facebook has been caught lying more than enough. So thanks for trying, but I have uninstalled WhatsApp and I'm happy with Threema.
Have you considered Riot (Matrix) or Signal? Both are open source so it's possible to verify claims made on their website, which is a lot less possible with proprietary software like Threema.
And with Matrix apps you can choose to run your own server. Not sure what legal ramifications that has, but practically speaking it allows the possibility of eliminating another potential weakness.
These vulnerabilities would then at least be bespoke, particular to a specific server, preventing mass surveillance. At least if you're not talking about potential vulnerabilities in synapse (Matrix server software), but then a strong security team wouldn't help that much.
