Develop in-app purchase for subscription process (both iTunes and Google Store)

We need to develop in-app purchase process for our app caracal.
caracal.life is a solution to monitor hypertension and prevent complications, using connected objects, synching data in a secure cloud, and facilitating information sharing with doctors and pharmacists. And users get rewarded when they use the service, with points that they can redeem in experiences like yoga classes, massages and more
Stack:
Backend : PHP5, Zend Framework 2 + Doctrine ODM (MongoDb)
Frontend web: good knowledge of HTML/Javascript + Angular
Mobile: Xamarin/C#

Current state of subscription:
When someone creates a user and registers in caracal, they get offered the first month of Premium subscription free
Shortly before the end of the first month, they will get email inviting them to subscribe to Premium, otherwise they are downgraded to ‘Standard’ status
In that email, there is a pdf attached, where they can click to a link in order to access the payment and billing platform
Every month the user will get an email with an attachment with a link to click and pay in order to remain Premium


Desired future state
We want a simpler, leaner, less cumbersome process, and a process that complies with Apple rules for online subscription
The user still gets the first month of Premium for free
Then the subscription is done using the in-app purchase process
In the User Profile, there is currently only information about the end of date of the Premium subscription. We need to keep that information but also add a line saying ‘upgrade your profile’ (in 4 languages, CA, FR, EN, ES) so the user can subscribe in the app (i.e. is pushed to the subscription process/API)
4 products are available in the Apple store, all are non renewable:
Premium 1 month
Premium 3 months
Premium 6 months
Premium 1 year
in Google store the product list is not created yet, because we need to update the app manifest first https://developer.android.com/google/play/billing/billing_integrate.html#billing-permission in order to be able to create the product
first create an apk with the app manifest updated that we can publish
in order for us to update the product list
The user should NOT continue to receive an email with the subscription payment attachment (current process)
The user should receive notifications before the end of the subscription, like 1 week and 1 day before the end date.