I suspect that something as simple as 2-3 emails prior to renewal, each with a one-click (no auth needed) clear, unobscured "unsubscribe" button would be sufficient.
Personally I really appreciate it when a service sends me an email or to reminding me of an upcoming renewal and making sure I have a chance to cancel. It tells me that they’re not hoping for or relying on users forgetting their subscriptions to make money, which imparts am image of integrity.