Healthcare providers operating a Matrix server won't be communication providers to the public, and if they aren't using a web client or some similar crap, breaking E2EE won't be possible. Webmail providers claiming E2EE like Tutanota are imho liars anyways, because Javascript on a website isn't a secure "end" for the encryption.

So use client software that isn't attacker controlled easily, keep your keys private, only encrypt to trustworthy keys and you'll be fine. Matrix ticks all those boxes if you don't use the web client. And the only legal way in for German law enforcement would be to infect your device with some trojan ("Bundestrojaner", like e.g. NSO Pegasus).

In this case law enforcement can simply request the data (as long as such a request is legal) at either end. No need to attack the connection in between.