> Seems like passkeys use a very simple model where you are using a single device with a single browser or are somehow syncing across devices with some cloud service - and from your description it sounds like that doesn't even work.

Unlike passwords, you can have multiple passkeys per account. You can have 5 passkeys for your amazon account if you use your amazon account on 5 different devices. If you lose device 4, or if it gets stolen, you can just delete passkey 4. The other ones are safe.

Or, you can use a syncing service like a password manager. Both solutions work!