I've been building the app with Flutter and Firebase for the past few months. Today I made the decision to move off Firebase and build a custom backend with Django Rest Framework.
This wasn't an easy decision, Firebase would definitely allow me to launch a first version quicker, but I am already seeing limitations with it. Mainly, I have complex relationships that can be modeled much simpler with a relational database like Postgres. I'm noticing to make this with with Firestore's NoSQL setup I have to copy data and keep it in-sync or read tons of extra documents which will not scale.
I have some experience with Python and Django but definitely will be learning a lot. It's exciting to me, at this point I'm digging deeper into the tech and "building this for the long term" which I realize could be a mistake because it will delay the launch.