> You could fix it by eg. `$state(undefined as unknown as HTMLElement)` but that's dumb

I think the generic is what you are looking for

  class Wrapper {
    dom = $state<HTMLElement>();
    constructor(el: HTMLElement) {
      this.dom = el;
    }
  }

the dom property will still be HTMLElement | undefined, if the 'undefined' bothers you have to add an exclamation mark and write "$state<HTMLElement>()!"