syncing passkeys around devices without issue?

they are syncing, but under no circumstances it let you see the passkey's private key in a format that you can import elsewhere, which reduce the amount of damage that can be done, but still if an attacker gain access to your Google account and its "password manager" (or any other password managers tbh) it's mostly game over at that point.

Personally I don't have all my passkeys on a physical device, they're mostly stored in my Bitwarden vault for the convenience of multi-device sync, and the important accounts that offers SSO into other services (Google, Facebook, Amazon, Apple, plus Bitwarden) are protected by multiple hardware tokens with a Passkey for redundancies.

Security is as strong as its weakest link.

For the accounts that are highly important, you might want to use only keys that are bound to a device like a computer, phone, or hardware security key. This would require a bit of manual management as you swap out devices and hardware keys but for a limited number of important accounts this should be feasible. For all the other general accounts, storing them in a password manager can continue to be the most convenient way to use them. The Google/Apple/Microsoft solutions take this second approach and allow them to be synced across devices.

As for the portability, it's still relatively early and I don't think there's a standardized format to export passkeys into. It's only a matter of time before things settle down and different password/passkey managers support importing and exporting to at least one format that will work.