Malware could go to the router and spread absolutely. However other devices firewalls should be pretty good at stopping that. Also I dont know how common that is for malware tbh

Signal. Requires little technical knowledge, other then knowing about end to end encryption and why its important

I would like to use matrix since if anything, I think its cool. But noone I know would be remotely interested.

You can add your GPG keys to the thundermail client, and it just sits there locally on your computer encrypting/decrypting mail as needed. Another caveat/weakness with PGP/GPG is it relies on the sender to actually encrypt their email with your key. If they don’t, of course the email will sit there in plain text.

As others have said, tuta. Could also use PGP, but that doesn’t hide the sender/receiver only the content