Yup let's trust Unity's crummy ways, keeping in mind that it's actually in their best interest to NOT detect false installs, since they get money when it doesn't...
If I had to do something like that, I would make it every time the installer runs, every time it's installed by a launcher like Steam, and as a fallback every time the game executable runs for the first time unless an installer or launcher hands it a key to say "you've been paid for already." But I'm by no means a game dev so idk.
Cool, find a game dev you hate and set up a script to install their game and run it once as many times as possible. Let that run on a machine you don’t use for a while, then drink their tears