[latchkey]

Firestore has very specific usecases that don't match a lot of things that Postgres is really meant to handle. Namely, as you are learning the hard way, it isn't a relational database. It doesn't make it bad, it just isn't what you thought it was. $1400 a month is nuts, it sounds like something isn't optimized well.

GCP Cloud SQL (Postgres) has been fantastic for me. Easy to connect to, easy to scale and not that expensive.

I have GCP Cloud Functions (golang) in front of that and they spin up, connect to the database and serve the whole request in under 1s. Hot requests are 80ms for submitting some simple data.

If you do it right, you can minimize persistent connections to the SQL backend (offload as much as possible to PubSub messages which a backend function can handle). This will keep your bills down too because you won't need as large of an instance to serve requests.