C# has indeed finally gained sum types this year… in preview. We’ll see if anything makes it into C#15 but they’re closer than they’ve ever been.

Based on your description, I think a good library for data structures that are intended to be used immutably would help with some pain points. Something with copy on write support or something like that. It's a powerful style, used it in haskell and it prevents a large class of bugs, almost down to just business logic bugs, which is what you want.

I like the style you're describing, thanks for sharing.