I might be misunderstanding exactly what you are trying to do (correct me if I am wrong!) but it sounds like you are trying to basically “fake” the client credentials flow with an actual user account. You could use a username + password instead but that is complicating the scenario… you’re doing a full OIDC login where you only need to pass a client secret.
In either case you need to store and provide a secret (client secret or user password) so I’m not sure what using a user account buys you in this scenario.