I am not your lawyer and this is not legal advice for you or anyone who reads this.
Nextcloud encrypts data e2e, so your point there is misguided and not really relevant. You can’t be compelled to provide a password/decryption key as long as it doesn’t exist as physical evidence. This is why lawyers advise clients to use a PIN instead of face ID or fingerprints; biometrics, like all physical evidence, can be subpoenad.
Self hosting services like matrix or email is a bad idea if you don’t really understand what you’re doing, like many other things. If you keep you stuff updated and are intelligent in how you structure your network there’s not really anymore risk here than paying someone else to host it. If you keep you stack simple and follow best practices, code and configurations written by industry experts do most of the heavy lifting.
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