I respect your choice and right, Tuta is not compatible with PGP, but that doesn't mean I (a Tuta user) can't send an encrypted message to someone on another service or receive one. In that scenario, there's just a bit of server code that users interact with in between to enable my encrypted message to be read by others outside of Tuta.
The process is documented here: https://tuta.com/blog/email-encryption-guide-how-send-encrypted-email

If the email's recipient is on Tuta, they just read my message in any of the available client apps.
I agree just supporting PGP as an option in Tuta would be better, but I think someone considering their options should know about the above. The current Tuta process works for me.