I have managed to get @protonmail notifications working on a GSF free @GrapheneOS phone by ... running the proton bridge on a @debian virtual machine and IMAP idle polling it with a shell script. The script then sends a request to @ntfy and boom mail alerts are back. Also that's geek bingo for me.
@JurassicPork Sure: https://github.com/jquagga/protonntfy Its a pretty basic script I mostly sourced from someone else but it works great. There are apparently some tricks to running proton-bridge on arm I think but I believe someone figured out how to do that as well.