I really appreciate you having a full unlocked copy of the game with up front pricing and trying to solve this issue in a thoughtful way.
In the old days, the free version would be a limited preview of the game, and would direct users to purchase the full game. We called it a demo or shareware, as in you were intended to share and copy it widely.
You could also have the “in app purchase” be the full game unlock.
Member it (southpark) :) one could learn some assembler by taking down those limitations and cd checks. Who would thought that it will be useful in reverse engineering malwqre in future? Hah...
Dont get me wrong, at that time very little ppl in my country had ccs to actually buy any software even if, they wouldnt give it to kids :)