hook a function after successful payment for a subscription

You want to use the pmpro_after_checkout hook. It passes the $user_id in and you’ll have to lookup the order and/or membership level yourself.

Here is the hook in action:
https://github.com/strangerstudios/paid-memberships-pro/blob/dev/preheaders/checkout.php#L1018

If you search for “pmpro_after_checkout”, especially on gist.github.com or paidmembershipspro.com, you’ll find some examples.

Good luck.