Motionsickness
Inside the battery usage section select the period of time where the drain occurred, right under the screen time there's an option that says "view by apps" tap it and it should let you change to "view by systems", there you can see the culprit of the drain (assuming that it really is the systems fault).
I can bet that Mobile Network is the main culprit, you can find other posts in this same forum about it and it has nothing to do with Graphene, the modems used by Google on the Pixels has always been topic of discussion.
You could still backup, revert to stock, make some tests and document the results, but I'm pretty sure it would be the same (it could even be worse since stock has all that bloat running on the background).