Have you looked at Sanity [1]? You get both a fast cloud backend (with a powerful GraphQL-like query language with joins, transactions, object-level security, text matching, real-time change notification, etc.) and a collaborative editing front end (optional, but recommended!). The latter is open source and modular. You can deploy serverless just like Firebase/Firestore. Gatsby is a popular way to serve Sanity.

Disclosure: I've contributed to their database tech, but I don't work for Sanity. Mostly just a fan.

[1] https://sanity.io/