Hey @Nic_Oliver,
The client credentials grant flow is specifically for apps where the app and store are in the same organization, i.e. a merchant creating and using a custom app on their own store. That flow isn’t available for partner-distributed apps.
For the agency/partner workflow, the path hasn’t changed. You create the app in your own Dev Dashboard, select Custom distribution, and send the merchant an install link. If you’re using the Shopify CLI app template, Shopify managed installation is already set up for you, so auth and scope updates are handled automatically on deploy. For non-embedded apps, you’d use the authorization code grant instead of client credentials.
For shopify app execute / shopify app bulk execute, those are development tools that work against stores where your app is installed. You can read more about their usage limitations and use cases here.