Have you tried Scala? It checks all the boxes, and is a mature language. The reactive UI runtime is provided by the library Laminar [1].

Technically its type inference is not HM but it's as expressive. In particular it has GADTs and HKTs, which I saw in your docs.

I wonder what you feel is missing from Scala (its .js/Wasm version) that Lumina provides?

[1] https://laminar.dev/