But is it the case that the Yubikey is essentially treated the same as a trusted device? What if I want to untrust my devices and only trust ubikeys (without removing the device from my icloud account?)
Yes but my understanding is that you can remove the Yubikey without possessing it, just with a “trusted device”. I want to mark all of my devices untrusted (wrt icloud account changes) and rely only on Yubikeys