[BackBlast]

I'll have to look at some of these, I've reviewed some of these but not all. You are missing some I'm familiar with.

PouchDB+CouchDB work well out of the box with minimal fuss for open pieces you can just plug into this role. PouchDB handles the client's state persist and replication on the client, couchdb is the reliable cloud service you can replicate to.

Meteor, at least their pre-apollo stack had realtime collab type features with their mini-mongo client and oplog tailing.