I need to be able to charge a deposit to reserve a subscription for future-dated services that are limited in availability in order to confirm bookings/reservation of those subscriptions.

Charging the setup fee on the scheduled commencement date (and only if payment details are provided by then) is not sufficient commitment from a customer to make a limited-availability service unavailable to other potential customers.

Ideally, this deposit would be an invoice sent out immediately with a configurable expiry date, so that the subscription cancels if the deposit is not paid within the defined period.