So I tried an idea posted by @delitako on this thread: (https://discuss.grapheneos.org/d/9968-why-tf-does-proton-mail-need-google-play-services-to-show-notifications/2)
"I have a crazy idea, but I've never tried it. Proton Mail has an option of sending a notification email to another email address whenever you receive an email. I assume it won't send full email contents but rather just limited metadata (PGP-encrypted emails still expose the subject, sender, and recipients anyway), so it shouldn't leak much. Then, if you can find a client that supports notifications for this email, then you can just check protonmail whenever you receive an email there. Most clients that don't use google play services would rely on polling or background connections though."
And it worked. I feel I might keep this as a workflow until/if ever Proton decide to not use Firebase for notifications. Although I assume there are security gaps, happy for people to elaborate share thoughts.