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.