You can control user registration and levels of privileges easily with firebase and some extension to get user’s gmail. Activation however must be done manually when you see the payment. (I am considering to create an app to see my different firebase projects and modify the users rights accordingly. )
One must be careful of GDPR issues also… some kind of consent should exist on initial run of the app about personal data kept such as the email.
You can even use firebase entries to set counters of days from initial installation for trial period and this could be bulletproof no matter if user uninstalls the app or clears data…
So based on all the above, unfortunately the starting point of the value is 0.
I am curious however for your method. If you wish we can further discuss it privately.