How do I reserve my App name whilst I develop the App?
Reserving a new App name
Upload an early build to the App Store.
Submitting to TestFlight should make sure (but may not be required).
When you declare an app in AppstoreConnect, one of the very first informations is the app name:
You have before selecting to check it does not exist already in Appstore.
That should be enough to reserve the name, long before submitting your first build.