You have two choices really. Either you go with a full JavaScript stack. This would be Angular on NodeJS or React with Apollo at the front end and GraphQL Yoga and Prisma on Postgresql at the back end. Your Web API here is GraphQL.

The other option is a mixed JavaScript and Python stack. I would start with React with Apollo for the GraphQL API talking to Graphene and Django on Postresql. I would later build in Flask once I know exactly what I want my business logic to look like and there is more time.

I would prefer the Python backend because it makes going into Machine Learning and AI at some point easier than on a JavaScript backend.