Subscription/recurring appointments. I can't customise the amount of time between recuring appointments

Hi, I run a mobile car wash business and want my clients to be able to schedule recurring appointments. There is the preset options of ''every day, every week, every other week, every 3 weeks, every 4 weeks, every month, every other month, and daily'' however no preset to schedule a custom day, as some customers want to schedule every 6 weeks and there is no option for this. I tried to create a subscription every 6 weeks which worked, however it doesnt actually schedule the appointment with the subscription, rather the payment of the subscription generates a code in which they can schedule an appointment. the payment and booking appt are separate which is just more inconvenience and confusion to myself and customers. Can someone please help? Thanks