If you really don't want GSF and Protonmail is the only app you need it for, you could check out Tutanota as a replacement if it fits your needs (they have their own implementation for notifications and don't require GSF).
If you need Protonmail but don't like GSF to run in the background for other apps to use, you can create a user profile only for Protonmail and use GSF exclusively there. Thanks to notification forwarding, you'd get a notification on your main profile but you then have to go to the user profile to actually see the content (if you do fingerprint unlock, this shouldn't be too much of a hassle).
Anyway, you'll figure it out and have a good time no matter what. Even in the same profile, Sandboxed Google Play Services are so much better restricted and controllable under GOS. Let us know if everything worked out for you.