> Seemingly not all services allow registering an additional key

I’ve only ever encountered that braindead design with AWS, every other place allows multiple keys.

And I can’t say I find multiple keys cumbersome, it’s simply the same procedure again: Click add, insert key and tap the button. Just twice instead of once.

Specifically, I want a backup key off-site. Anytime I enroll a new service, I need to retrieve my backup key from storage for the purposes of enrolling. Depending on how/where the backup is stored (bank box, parent's house across the country, etc) that would make the system less workable in practice.