RE: Membership Method or ISV for BC
In cases like this i have usually ended up setting up the members in Dynamics Sales and only posted summarized member fees into Business Central.
That is simply because there are a lot of regulations on when and how you should post a member fee request. Here we can not handle them as posted sales invoices due to accounting regulations. So we send the member fee request from sales and just post the income in BC when the member pays it.
So we update Sales with individual member payments and just post the sum to BC.
You need to find a solution that match your accounting regulations.