My recommended Substack policy: First, establish the maximum monthly amount you’re willing to pay. Then, choose monthly rather than annual subscriptions. When you reach your max and want to subscribe to a new Substack, you can only do so if you unsubscribe from one of your current ones. Gradually you’ll converge on the ones that are essential to you.
(You could do this will annual subscriptions as well but it would take you a lot longer to achieve that convergence.)