For now i use sqlite to deal with transactions and only make backward compatible updates to structs. Brittle, but it is a toy app anyways.
(Normally use django to deal with models and migrations, but wanted to do something different)
I gave it a good go to use mongo and firestore for a few projects, but after a year or two of experimenting I'll be sticking to SQL based DBs unless there are super clear and obvious benefits to using a document based model.
* meaning, when there's enough code that depends on it that changing it would require some planning
Though most of the time, in that situation, I would pull it out.
But I tend to just use Django. Every time I try piecing together the parts (ex FastAPI, Pydantic, Alembic, etc) I reach a point where I realize I’m recreating a half baked Django, and kick myself for not starting with Django in the first place.