We wait a few weeks before clearing out the data after uninstall. Its still within our 30 day privacy policy window but not so fast that if the customer regrets the uninstall that they can’t reinstall and find that all of their data is still present.
When the cloud site is destroyed (post deactivation for lack of payment) we clear out the data immediately because we delete the product data at the same time.
Currently there is no accessible mechanism to clear out app storage that admins can access: by design.
The only real way for you to clear out the data would likely be a web trigger that you can spam that scans for keys and deletes them from app data.
I hope this helps even if there is no ready solution.