When Firebase was first built, using a document store was a great choice for building a local abstraction that enabled optimistic updates and offline mode. But the lack of relations makes it a real schlep to change your data model when you start adding new features to your app. You end up hand-rolling joins or duplicating your data to avoid complete re-writes. [1]
One of the first things I've found myself doing with the few firebase realtime database based projects I've made is some kind of abstraction to emulate joins.
I realise that this is probably a symptom of "holding it wrong" and not embracing denormalization, but it was always present and also horrifically inefficient w.r.t to firebases pricing due to the amplifying effect it had on reads - none of those projects would've been practical to take to market without reworking that significantly.
Really what I wanted was a relational database that also did the fun/flashy real time updates without the heavy lifting. At face value it sounds like you're offering exactly what I wanted so I look forward to giving it a try next time!
With Instant, you get a relational Firebase.
[1] https://www.instantdb.com/essays/next_firebase#firebase