I would like to purchase the pro version to use playlists but since I’m using Graphene OS on my Phone without any Google Accounts I wanted to purchase the app another way. How can I do it? The Troubleshooting page says “The Pro version will only work on Android devices where this email is registered as an Android account.” which is not possible with Graphene OS.
Just install a email app that register the account at OS level. There’s a few post on this forum that gives ways. Don’t know about your specific ROM.
Unfortunately it is not possible to sync Protonmail or any other Mail-App with Graphene OS as far as I know. Is there another way to “go pro”?
Well you did not search I suppose then 
Anyway no there’s no other ways, this one is already a lot to maintain and avoid Google issues.