yes it is sad, I have worked days and nights for this and I am not being able to reach more users because of the quota on the apps.
For premium users sync every minute.
Use Indian payment gateways if possible, though Paypal works fine too.
This is by far your easiest and clearest path to monetization, at least that I can see. And it seems you're long overdue for monetization.
Logins, refreshes, clicks, new things synced, etc... can all update the last activity timestamp.
Kudos for assisting, its just frustrating to realize its needed.
E: If you haven't already done this, of course.