PGP can be a real rabbithole if you get into the gory details of what subkeys are, and what exponent you should use and so forth.

Fortunately Thunderbird has PGP support built now ehich makes practical usage very easy after the initial setup. If you are public enemy #1, then you'll want to go down the rabbithole and learn about all the ins and outs of PGP. Honestly though, the main risk is that someone you talk to is compromised/a snitch/turncoat/enemy of some sort.

As for Matrix, some Matrix servers require an email to register, some ofvthose also won't allow disposable email addresses (mailinator, sharklasers, etc.). This is because of spammers. I found a small community of hackers who run and instance and just asked them to create an account for me. If you are on Mastodon check out the "tildeverse". Those folks are likely to help you out. Just don't be an asshole and ruin it for everyone.