I'm also not a fan of being limited to certain cloud services (I use neither Google Drive nor Dropbox).
Good news!
Boox will run any cloud sync that has a recent Android app available. It's just an Android tablet, at heart, and it's already unlocked.
In particular, the local NAS sync client for Synology runs like a dream.
Edit: Oops. I missed that we switched from Boox to reMarkable, there.
I guess I'll leave this since Boox sells eInk tablets that are feature matched to reMarkable, anyway.
I don't believe in anything supernatural, but I'm pretty sure wiper blade attachment designs were somehow still born in the deepest fires of hell.