If I were you I would get a clean phone, instal the Fly App on it and try to log in to the account you used to bind the equipment. Leave the iPad alone for the moment - until an iPhone or iPad user chips in.
If you can log in to that account on another screen device then you should be able to unbind the equipment. I have unbound a
Mini 2 using a different phone from the original but the second phone was logged into the original ( binding ) account.
I can't imagine why simply deleting the App from a screen device would prevent you logging in to the original account on another screen device.
I've deleted the Fly app from phones several times and not had any problem using the original account on another phone or, for that matter, reinstalling the App on the original phone but don't try a reinstal on nthe iPad just yet.
Did you, if it is possible, close the account?
Sorry I do not know what happens on Apple stuff when you delete an App but with Android stuff, although the App was nominally deleted the actual data storage file system remained behind .... or that was the way it was when I last deleted a DJI app from an Android phone. I could see the remaining file system via a Windows compter.
However I do not know if that has changed with the more recent Android versions.