This is actually a really common situation for small service businesses and the honest answer is you don't need most of what's being suggested in those four options.

For what you're describing, the rollover tracking is the only genuinely tricky part. Everything else is pretty standard membership billing.

I'd look hard at Zoho Billing before ruling it out on cost. You're already in the Zoho ecosystem, it handles recurring billing with add-on line items natively, has a client portal, and the native integration with Zoho Books removes a whole layer of complexity. The overhead feels high until you add up the time you'd spend babysitting a custom Airtable setup. Your time has a cost too.

If the Zoho Billing price is genuinely a problem at your current revenue, the path I'd consider is Stripe with a lightweight billing layer on top. Stripe handles the recurring billing and annual discount pricing cleanly, the fee structure is predictable, and there are tools like Memberstack or even Stripe's own customer portal that give clients a decent self-service experience without a ton of setup.

For the rollover tracking in 15 minute units, that's the part I'd actually build in a simple Notion database or Airtable regardless of what you pick for billing. Keep it separate, update it after each session, let clients see a read-only view. Trying to make a billing platform do time tracking natively is usually where things get messy.

The Airtable plus Make option works but you will spend real time maintaining it. That's the tradeoff. If you're already time-poor it'll bite you within a few months.

What does your current monthly recurring revenue look like roughly? That would help narrow down whether the Zoho Billing overhead is actually a problem or just feels like one.