To switch from JS to TS I’m willing to give up absolutely anything. I’d do five minute build times per small source file to do TS.

TS is probably as good as transpilers get. It really is quite good. I like that it maps one ts file into one js file and creates readable code (although output is normally bundled so readability is lost). I like that ts aids dev discovery in tools (intellisense) at write-time. But ts still needs a build step, and that's a big down-side to the immediacy of "plain" javascript.

If browser vendors bundled tsc or its equivalent then we'd both be stoked.

Not exactly this, but there was a proposal that would allow for typescript type hints to be considered as normal JS syntax: (in the same way python PEP 484 – Type Hints are considered optional type hints)

https://github.com/tc39/proposal-type-annotations

Deno on the server side is pretty cool and uses https://swc.rs/ (Speedy Web Compiler) - doesn't take forever to start, but kinda implements typescript as if it was just a type annotations without the more in-depth actual type checking, so it is possible.