My $.02, start with a microservices design first for all new projects. Think of ways to divide up many services into functional backend requirements.
Now I'm your case ask, what backend services do I need?
It sounds like the only service you need is something to service static web content. For that, you can just use azure storage, or nginx. This will perform better than rails or node or even .net core for static (unchanging, like spa js and html) content.
Now if you need to do backend processing for persistence, you can add that later, and your architecture hasn't fundamentally changed. Each new distinct functionality you can decide how to implement. Your APIs can be 0-unlimited, growing as your demands require.
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