> even though TS has its own share of problems with its 'kitchen sink type system'

Eh, people critique the kitchen sink type system but at the end of the day that system is why TypeScript took over, not a weird add-on you have to put up with to get it. It's what makes it possible to assign comprehensive types to ~every JS library, which is what allows TypeScript to gradually eat the whole JS ecosystem.

Any other approach stops short of the goal of being JS+Types, which means you end up with a different language with different semantics, which is where Dart failed.