yup, and if you want to really optimize it you could use functions as a service like lambda as your react rendering layer, which forwards to heroku or fargate or beanstalk or vanilla ec2 or whatever for your backend.
and if cloudflare workers ever support metered usage you could use that too for the react layer.
and if you want hyper performance you could use lambda@edge and intelligently route to your backend running on fly.io to minimize the distance. the somewhat unsolved problem there though is multi region master master replicated databases which are cost effective.
Ya it really depends. I like redis and Postgres generally speaking for data, and Ruby on Rails generally for business logic. And you can certainly do universal rendering with that easily with react_on_rails. If you do end up needing more perf, maybe go would be interesting.