Why would you take one passkey and move it between devices? Generate a new one. They're fungible. You set it ask to the exact same way you do today. It's not a problem.

Again, do all major sites allow you to register 6-10 passkeys? Not asking if they could in principle, but do they in practice do it? And how easy is it to log in with a new device to generate that passkey? Do I have to jump through hoops on my laptop, second phone, secondary browser, and so on?

Finally, if it is easy to register a new device, how does the anti-phishing still work? Can't an attacker just convince me to use whatever means I would normally use to register a new device, instead of an existing secure passkey?

This assumes all services let you generate new passkeys with no hassle, which is not true.