For the email setups we do for customers, it’s Debian OS, FOSS Mailu on the backend, and any client can be used such as Thunderbird which has PGP. K9 for android. Also we put on the VPS the web client SnappyMail. Having a web client is less of a big deal IF you’re running the VPS. SnappyMail is a hell of a lot faster to load than Proton, and the tools are decent.
If you want guidance on the setup, where we have no access and just tell you commands, we can do that as well, https://simplifiedprivacy.com/email-cloud-combo/index.html