I would have gone with

- React + Redux on AWS S3

  - Redux: You know you will eventually need to add it, so just do it to avoid needless workarounds before you give up

- Python with Django+DRF on AWS ECS

  - Admin interface to inspect your data + neat REST API with one line

- Postgres on AWS RDS

  - Plays very well with Django, new features are implemented in Django as soon as possible

Might make sense to think of cpu-intensive jobs such as image operations etc. I personally would have offloaded them to AWS SQS + Lambda running flask deployed by Zappa(Had nice experiences with Zappa+Flask)