Hello, I have this problem where the current Steam Account that I have cannot be verified for Cross Save because I don't have access to said Steam Account. I'm unsure if there's any other way to verify, but I'd greatly appreciate it if someone can help me.
You'll need to work with Steam and very quickly if you plan on unlinking it.
You can only disable cross-save if you can authenticate ALL accounts linked, if you cannot authenticate a platform you have to work with customer service to restore access to the account. Bungie will not forcibly unlink an account or deactivate cross-save.