This gap is filled by the likes of Couchbase where a single org controls the majority of the stack (spoiler/disclaimer alert: I've been working on Couchbase's Sync for 8 years)

You get local the document-level atomicity for sync. Multi-document transaction support on server side, KV access, SQL inside JSON docs or even across multiple documents, Full Text Search, and fine-grained/RBAC for document-level synchronization - but the cost is as much lock-in as it is financial. You can't mix and match storage, query or sync without pretty big tradeoffs.