Oh. Right. A plugin could just catch whatever webhook the subscription plugin made and plugin settings could map the subscription name to the group (and match user but email address). A group custom field could hold the associated subscription ID and move the user in ou out of the group when the subscription was updated. If it used a subscription name rather than id, a single custom field could handle multiple payment handlers.
The admin would be responsible for setting up the payment system and arranging for the user to get to a payment page.
Here is the webhook info for lemon squeezy
I’d love to do this for someone who had a budget.