"Supabase is an open source Firebase alternative. Start your project with a Postgres database, Authentication, instant APIs, Edge Functions, Realtime subscriptions, and Storage."
Budibase is interesting too.
People seem to like Appwrite as well.
I hear you on firebase/NoSQL. It's never quite made sense to me when people select non-relational databases to spend an extraordinary amount of time and effort to make it into a relational database. It's more work that just learning relational databases. I'm learning more about knowledge graphs that are not strictly programatically managed and it's been fun so far.