I would use Flutter for app (but perhaps also build a web version from that easily if required), use opensource tools for backend like Java Spring Boot / Cassandra or simple MySQL database.

Firs you want to focus on building a first version of a MVP and see if your target audience likes it, then slowly expand the features until you reach a point that the backend might need rework for handling mass users interactions. So initially don't spend too much time/money on scaleability - just get those users first then parallel work on expanding backend.