As for backend. Firebase is easy to use and quick to get started with. That said, it can get expensive pretty fast and I am not a fan of the lock-in it comes with it. I fully admit I prefer AWS over Google, although we use both today. However, even with AWS I avoid DynamoDB and vendor specific tooling that I couldn't run on a box myself. I'll use RDS for Postgres but that's because I can always move that to my own servers if the need arose. If I was using Dynamo that'd be tough. I'll use their API compliant tooling like MQ or their Mongo compatible document store if I wanted, because again I can move databases without changing code.
And I'd like to stress out the Infrastructure as Code as much as possible, so you can always keep track of WHERE and HOW runs WHAT.
I'd go for GraalVM or Laravel and Postgres as backend, as others have already suggested