Last time I used thunderbird was many years ago, like 10 years or so. Can you create a mail adress there? How does it compare to other providers like Tuta, Proton, Posteo and others?

Doesn’t Matrix have a huge problem with metadata or am I not up to date with this?

I’m using Tuta and their app for a few years now. The app was slow indeed but it’s good now, no problems so far. Lack of IMAP support is justified with security, they say. I personaly don’t need IMAP as I’m completely satisfied with the app, which is available officially in f-droid btw.