KeepassXC is free, open source, and supports passkeys. You can locally store your encrypted password vault wherever you like, and transport it between devices using physical media if you like (or self host your own personal storage synchronization server and sync your passkeys between devices like that).

No need to be a part of an 'ecosystem' to use a password manager or passkeys.

Too late, you're using web standards, you're part of an ecosystem.