Signal is probably best, since it has volume to hide in, and it's difficult/ impossible to screw up. Partly because it's centralized. Which is the problem with e.g mateix protocol - each node can configure thi gs a bit differently. Also why you should use the official app imo.
This is a decent walk through: https://www.privacyguides.org/en/basics/email-security/
But simple duckduckgoing can provide much more
https://5blockchains.com/posts/pgp-vs-signal/
https://ubos.tech/news/pgp-encryption-security-crisis-why-cryptography-needs-modern-alternatives/ (feels a bit clanker like - but info is legit)