Given that you don't strictly need to have one passkey per site, is this desire to move passkeys around a holdover from wanting to "export" your passwords? Because if you can export them, an exploit can too. I find passkeys rather more interesting when they cannot be exported from a HSM / key enclave / yubikey, but of course I need to be able to register multiple yubikeys per site, and a few of my accounts didn't allow for this so I ended up using my yubikey for TOTP since I can have the same seed on multiple devices.
My keepass database has around 400 different entries in it. If I needed to transfer to a new password manager, it's not feasible to go around to 400 different sites to register new passkeys. In case one might say the answer to that is oauth, I'm also not interested in putting my faith in Google/Microsoft/Apple being benevolent arbiters of my ability to access my accounts.
You could put your faith in LastLogin: https://lastlogin.net
You should be allowed to keep your passkeys in such enclave. But there seems to be no alternative. I'm in the same boat as the GP. I'm not touching passkeys unless and until I can export them into a file I can get my grubby hands on. I'm guessing that's never happening. Not sure what one-passkey-per-site has to do with it.
Export is a good check against lock in. I just went through my password manager and I have 60 passkeys. It would be a huge pain if I have to switch to a different password manager and there isn't export/import.