If you already know Ruby on Rails, do that.

If you already know Python well, do Django or Django REST.

If you don't have any prior language affinities, go Javascript/Typescript on the whole stack. Language switching is a real problem for me.

I'd stick with App Engine/Heroku for prod. I like Postgres alot, especially with JSONB. Since it functions as a MongoDB-like database if needed.

If App Engine fails you, you should be able to switch to another provider very easily, or a standard VM without a code rewrite only a CI/CD rewrite

With that being said,I run a monorepo. /api folder has Feathers JS with Postgres /admin folder has react admin website /website folder has public facing react website.

Everything can run local during dev. Everything is CI/CD piped 12 factor styled One git push and merge to master, deploys everything. I love it.

If a single provider fails me, I can switch pretty easily.