We have a Magento extension from Aheadworks (subscriptions for M1)
We want this to be editable in the cart i.e they can changed it to every 3 months etc. These options are in the extension but it won't let you change it in the cart or checkout.
We have a test environment for this, once finished may need help to move to live.
The screenshot is kind of small but can provide clearer if needed.