This is my fundamental problem with passkeys: I don't want to use any syncing service.
To be clear, I don't want to deprive other people of the ability to sync their credentials; I simply want to opt out myself. I just want to be able to manually back up and restore my credentials, like I've always done with passwords, but the passkey vendors seem to want to refuse to give anyone this ability. The vendors claim that this is to make phishing impossible, but I abhor paternalism in all forms, and also it's suspicious that this paternalism forces people to use the syncing systems of the passkey vendors, which are usually paid subscriptions. So passkeys become an endless supply of money for the vendors.
It's very telling that passkeys were designed and shipped without any export/import mechanism. You can plainly see the priority of the passkey vendors, which is to lock you in. Allegedly, export/import is coming sometime in the future, but I strongly suspect that they'll end up with some kind of "approved provider" system so that the big passkey vendors can retain absolute control and avoid giving power to the users.
I am also slightly paranoid as a security engineer, and admit that whole heartedly.
I wonder if there would be a way for vaultwarden to wrap passkeys such that a hardware FIDO2 key is needed to decrypt them "per-use", and prevent software on the host from stealing a pile of passkeys that give direct access to accounts without further MFA.
Right now it feels like passkeys in the password manager is akin to storing MFA seeds and recovery keys in the same password manager...
I wrote a quick PoC using certificates to encrypt a password, with the cert private key 'stored' in the TPM, with a PIN. This is pretty easy on Windows, which exposes the TPM as a special crypto provider.
If you wanted to go a step further, you could use a smartcard with hardware PIN reader as a PKCS11 crypto device, and use that to decrypt the long lived keys in the store, then pass it back to the host encrypted by a platform-protected key to be decrypted and used.
If you could get the right implementation specifics together, you could likely then have the smart card simultaneously re-encrypt the credential with a key bound to PCR state of the TPM via a policy. You'd then decrypt that ciphertext on TPM without a PIN, but conditional on PCR state of a couple of PCRs that represent your system like the secure boot toggle state and allowed CAs.
That lets you be a bit more "cross device" than a fully TPM solution does, though your certificate technique works fine as long as you keep an offline backup for enrollment if anything changes on your system.