Content subscription model through IAP

Hello

We're developing an app similar to patreon.

The app will allow creators to publish their content on the platform and users to see that content. The content will be available only through the app. Users also can subscribe to the creators and get access to some premium content.

So my question is how can I set up this type of payments through the IAP or how can I justify direct link to outside payment form like Patreon.

Standard recurrent IAP's are not suitable here. The user can have different amount of subscriptions of different prices, so every month the sum will be different or it will be several different payments.