How can I delete a custom ringtone exported from Garageband, after Garageband has been reinstalled?
FWIW I created a test ringtone using GarageBand on iOS 12.2 using the directions linked in your post.

I then use iExplore to delete the file under Media > Purchases where there were three files on my device at that location, a Ringtones.plist file that had information about the .m4r file which had a GarageBand icon and a com.apple.ToneLibrary.lock file which was zero bytes in size.

Since this was the only file I've created with GarageBand and was indeed the file I created and was the only one in the Ringtones.plist file, I deleted both, leaving the com.apple.ToneLibrary.lock file, and then checked the iPhone and the ringtone was no longer available to be selected in Contacts or System Preferences in iOS.



Note: I am not affiliated with the developer of iExplore, Macroplant, simply a user of the product.

This was also done with just downloading and installing iExplore without purchasing it, as I currently do not need the paid functions.