The double-click to confirm an App Store (or other) purchase is the least of your problems.
It is a common misconception that an iPad (or iPhone) can remain fully functional with damaged or defective physical buttons.
Whilst Accessibility features (such as assistive touch) can be used to augment the User Interface, these features cannot be used as a substitute for physical button sequences that must be used to trigger system recovery actions. Sooner or later you will encounter a situation from which you cannot recover.
Until repaired, you will encounter many difficulties - and may even encounter a condition from which you cannot recover.
As a temporary measure, accepting that you will still need to have your iPad repaired, you may be able to substitute TouchID/FaceID (as your iPad may be equipped) to confirm a purchase:
Settings > TouchID|FaceID & Passcode > [Use TouchID|FaceID For] - enable as required