How do I track client sessions and payments using Zapier, Wave, Calendly, and possibly ActiveCampaign?

I’m a coach that works with 1:1 clients. Current process:

1. Client schedules a free session via Calendly.
2. Calendly tags it in my CRM ActiveCampaign.
3. Free session happens. Client wants to move forward. I create a recurring invoice via Wave that autocharges their credit card.
4. They use a different Calendly appointment to schedule all sessions going forward.

My pain point/what I’m trying to do is twofold:

• When an invoice is mark paid in Wave > record somewhere (??) that x number of sessions are available. (Most often case scenario: they pay for item X in Wave each month that allows for them to book 2 sessions that month, so I need placeholder 1 and placeholder 2 to end up somewhere).
• Once they schedule a Calendly appointment, it throws the date in placeholder 1 above. Once they schedule another Calendly appointment, it throws the date in placeholder 2 above.

This process repeats. I have a way to track clients and dates. If someone reschedules or cancels a Calendly appointment, it erases it from this location.

Example of final output:

Client Jane Doe

Session 1 Placeholder: 12/1/20 scheduled via Calendly

Session 2 Placeholder: 12/15/20 scheduled via Calendly

Session 3 Placeholder: 1/1/21 scheduled via Calendly, rescheduled to 1/3/21 via Calendly

Session 4 Placeholder: 1/15/21 scheduled via Calendly

Session 5 Placeholder: [slot paid for but nothing scheduled yet so it’s blank]

Session 6 Placeholder: [slot paid for but nothing scheduled yet so it’s blank]

I would prefer this in ActiveCampaign, but I can’t see a place on a Contact record to store this. I’m open to an outside app, but the closest seems to be expensive ecommerce order management apps (too expensive for my case usage). I’ve begun to try Google Sheets, but woof! This is complicated, it seems?

HELP!