Very interesting and fun read, but I think you missed one way to be VERY accurate, as long as you don't mind deleting your users' batteries, looool.

Inside of a web worker:

const t0 = performance.now();
while (performance.now() - t0 < 250) { }
// done

I'm actually morbidly curious how well this would perform in the different situations you measured, hahaha. I even made a silly little codepen for it

https://codepen.io/LydianLights/pen/GRqLqrx?editors=0010