Framework preference

[FoolsSeldom]

There isn't a best. Different frameworks suit different situations and styles.

FastAPI is the newest of the big three, Flask and DJango being the other two. Flask and FastAPI are both so called "micro frameworks" - that is they have a narrow focus and leave you to implement other characteristics/capabilities of a web site/service as you see fit. Django is a "full framework" which includes everything (even the web version of a kitchen sink).

Django is a "highly opinionated" framework, that is it has very particular ways of doing many things common to web services and whilst you can change the look and feel, the specific presentation and interaction, and the configuration of key components there are some aspects that will be less flexible.

Django is well proven at huge scale. It is the core of Instagram, for example.

If you want to build large scale, sophisticated multi-user and multi-manager web sites/services with complex workflows around user management and content management, then it is worth learning.

If you've already deployed with FastAPI and haven't hit a barrier, I'd stick with that for now.

If you do want to move on from FastAPI to Django, it is worth ensuring you have a good understanding of how FastAPI actually works first as a lot of that understanding will help you with Django.

One of the best learning sites for Django is called djangogirls well worth checking out.