Here we can offer buttons like cancel go back and so. If you go the way I propose, the payment request page can be very small page like follows we maybe can use for all the payments (yearly and once) or we make different pages:

Vertragsbestätigung

Here we simply need a confirmation that the contract is done. User can get access to premium group right now. Invoice we sen manually until it ios not possible otherwise. Needed is a permanent access where we send invoices every year and where the contract must be dismissed in order to stop invoicing. Seems right now there is no other way to manage invoicing and permanent access.:
Sie haben den Premium Zugang für die jährliche Zahlung gewählt für Vorname, Name, email.
Hiermit bestätigen wir den Zahlungseingang iHv xxx EUR von Name am ... für jahr. Zugang endet am ...

Willkommen in der Gruppe ...

In English: You have choosen the premium access for yearly payment for full name, e-mail. You will receive an invoice yearly. If you want to stop membership you have to dismiss 1 month before the period ends. Access will end in .... what to do to prolongue ...
Welcome to the group XY
This text will be changed for sure later as well.

Wenn der Zahlungsvorgang abgeschlossen ist ...
Some additional text like thank you, advise if something goes wrong shoulöd be added. How the process goes on, if user can go back and so on