@CallMeWeezy
Instead of handwriting letters, you can continue using your e-mail and protect it with GnuPG. GPG allows you to encrypt your messages so that only the intended recipient can read them. This way, you can maintain the privacy of your communications without being forced to resort to analog methods.
https://emailselfdefense.fsf.org

If you are looking for a way to communicate synchronously and in a freedom-respecting manner, XMPP is the ideal option. This protocol is federated and decentralized, allowing you to choose from multiple servers and ensuring interoperability between users on different servers. You also have the option to create your own server.

You can encrypt your conversations using GnuPG (XEP-0373) or OMEMO (XEP-0384). XMPP also offers multi-user chat rooms.

For Android, I recommend the Conversations client, which is very user-friendly. Conversations are automatically encrypted with OMEMO by default. As for GNU, there is a variety of clients available, including comprehensive options like Gajim and command-line clients like Profanity.