> some sort of "TypeScript-ish" language that, if a TS programmer plays by a stricter set of rules

This is basically just rust, no? As a TS developer, I've found picking up rust to be really neat.

Try C# instead.

I write a fair bit of C#. It feels like the prequal to Typescript. Its lack of discriminated unions and type narrowing feels like a step back from Typescript.

Dunet is really good.

https://github.com/mcintyre321/OneOf

https://github.com/domn1995/dunet

It's on the roadmap and will arrive at some point:

https://github.com/dotnet/csharplang/blob/main/proposals/dis...