If my passkeys are spread across dozens of vendors
You're picking what vendor you want, not picking unique vendors for every unique passkey.
Can I get my passkeys out? Can I migrate them freely between any vendors? Can I take backups for myself from all the vendors? Can I restore the backups into each vendor? Can I replace those vendors with a self-hosted solution?
It depends. It sounds like you just don't like or want hardware-locked/TPM passkeys. If you generate that type of passkey, then no you can't do much of that. Maybe your major complaint is that some major OS's default to hardware-bound passkeys?
My reference to "vendor" meant to include self-hosted/FOSS solutions.