Which is a shame because JavaScript is constrained due needing backwards-compatibility above all (among other issues) but TypeScript isn't

Not much of a shame IMO - there are plenty of other languages that transpile to JS without those goals, if you don't care about backwards compatability and 1:1 mappings to vanilla JS.

On the other hand, there's only one language that I'm aware of - TypeScript - prioritizing these things, which is a major selling point of it to me. I don't want yet another leaky abstraction, or yet another runtime heavy enough for those abstractions to not leak.

I just want enough types for my intellisense to work right, and for my sorry ass - spoiled by years of nothing but static typing systems - to be able to code without drowning in a veritable sea of uncaught type errors to debug, when I poke frontend stuff.

Typed and those features? there isn't a single one. But yeah, maybe what I need is a fork of TypeScript with those features (or maybe some sort of plugin)