Need a hook to be added to an existing plugin (new request)

General information for the website: Non profit organization
Kind of development: Customization of existing website
Num. of web pages/modules: 1
Description of every page/module: We're running a Wordpress site where members have an account (buit-in WP account plus some custom WP meta).
Each user is a member of the non profit organization (NPO) and pay an annual fee for their subcription.
The fee is paid using a plugin called WP Simple Pay Pro 3, which then carry the payment to Stripe API.

I provide a copy of the WP site with the DB and test keys for Stripe so you can work offline.
Description of requirements/features: My needs are (I hope) rather simple to understand.
When a logged in user is making a payment, I would need a hook (a plugin) that updates user's meta data (such as payment date or payment amount).
That is the basic. Now here are the challenges:

1. Some calculation has to be made in order to update the due date for next payment.
2. We're using several forms for payment (one per language), so it's for all payments through Stripe.
3. A counter called Seniority is incremented every time a payment is done (but once a year only).
4. A single user can pay several subscriptions or several times, so some counters are incremented.
5. The current year is calculated from August 1st to July 31st (and not from January 1st).
6. I need a very short alert with details to be mailed to an admin when the payment is done.

Note that we're in Japan, GMT+9, and this may impact communication (not always real time).
Specific technologies required: Wordpres plugin, PHP, Wordpress DB (usermeta), Stripe API
Extra notes: This is a new request as my former developper (hired here) left me without feedback and without a single line of code (although he had one month to do the job). I selected a skilled programmer rank in the top 5% of PPH but I guess I was out of luck (he had only two works completed with 100% feedback, so I wanted to give him a chance).

Please, when you make me a proposal, be specific about your suggestion to answer my needs.