Objectbox is an alternative, if you are looking for one

It's the only alternative I've seen so far that is like Realm in that it has an open source local DB and a paid sync service. But I fear the same fate as Realm, since it doesn't have an open source sync component

Ditto is a very viable solution, we offer device sync even without a central server. Ditto is a cloud optional edge sync platform that can sync data between devices, even without the internet. We are partnered with MongoDB to provide an alternative to Device Sync. I encourage everyone to read our docs:

https://ditto.live/