[Herbert2]

General answer since this is a general question.

DB engine: Postgres, something more exotic (dgraph, cassandra etc) depending on use case.

API layer: Microservices with grpc, write services in languages which are/will be good for each service's use case. Allows for a relatively easy longterm upgrade path, service rewrites in new languages when staffing or needs require.

Frontend: React, because a) it's great and b) you will worst case be able to reuse a lot of techniques if you decide to use React Native, best case a lot of code. Structure the frontend with lerna and you can avoid the churn of JS build tooling for more stable parts of your application.

Caching: Redis or Memcache

PaaS: I like GCP more but they're pretty much the same dish plated by different chefs.