I agree with all said here.
If I were to start with plugins again, I would simply add a nagging screen to buy, that appears after 30 seconds and reappears every 5 minutes or so. The user needs to click that away, and has to buy a “pseudo secret” code the users have to buy and enter.
That code will be shared eventually, but at least you get something from the honest users. This also helps to gauge interest without investing too much. You will see how much use there will be.
Pro-Tip: add some analytics (anonympous counter, careful with GDPR) and a field to signup to your newsletter, that might turn out to be even more valuable than the income…