You will not use the same passkey for multiple application/service.
You will generate a passkey per application/service.
I will certainly though not disagree on security... if that is your thing then do not sync private keys around, but the tradeoff is always there. If security would be critical, I would favor client certs with smartcards, but browsers do not support that "too well"
If I have "n" private keys for an account, I can use another private key and revoke the lost/compromised one. It's that simple.
Your secure enclave is not much different electronically from a smart card with a biometric password actually. People think Passkeys as SSH keys on disk, but it's more of a long private key on a single-way secure enclave. This is why people cry "platform lock-in". It's platform lock-in, but it's a secondary effect. It's actually a "proper HSM, but integrated".
Providers will most of the time allow to register multiple passkeys or other authentication means, hopefully ;-) which has its own downsides.
I am well aware how the internals work of keystores. But the benefit with "client certs" is that on mTLS you get added benefits besides where the key is stored. And that is that you can "prevent" mitm attacks.
But I guess that is a subject for another thread.