The ProtonMail Android app is open source

https://protonmail.com/blog/android-client-security-model/

Not available on F-Droid yet, so no option for me

You can get them via protonapps.com

Well i suspect that with the next realase of ProtonMail, the app will be for download on Github. Which is where i get my apps if they are not on the F-droid main repo or Third-party repo. That’ sat least what they did with ProtonVPN