Sounds like you mostly had experience with old style server side development (like php with no frameworks maybe?)

Have you tried building an app with a more modern framework, like Phoenix or Blazor? You won't even need to write a single line of javascript. All business logic is on the server (no duplication of models) an can be easily testable.

In my experience, SPAs tend to get overly complex, and they mix (and duplicate) business logic with UI logic.