If you're using AWS anyway, I think using their managed Postgres / RDS could make this a lot easier operationally. No need for custom syncing/backup scripts, easier to scale the storage up.

To the larger point, while I totally agree with the premise -- most apps will never need to scale beyond one large instance, I'm not exactly sure what's the actual tradeoff is. If you're writing a simple CRUD app, it is not really controversial that it shouldn't need any complex distributed stuff anyway, it is just a simple python/Go app and a data store.

Most "fancy" things outside that single process paradigm, such as avoiding using local disk and using S3/RDS/document stores, using containers for deployment, etc. usually have more to do with making the app operationally simpler and easy to recover from instance failures than scaling per se.