Oh yeah, this is my area of interest. I've built a lot of member management systems over the years; in Drupal and prior to Drupal.
Currently I use Drupal/CiviCRM which centralizes member, events, groups, commerce, accounting, communications and the all important reporting. There are are other activities like campaigns and pledges in CiviCRM but I don't find the need for them at the moment.
One area that is really important is related contacts. So you have individual contacts and organization contacts. The individual is related to the organization in an employee/employer relationship. Have you looked at this from a Drupal centric point of view.
Great work. I'll be looking at this in more detail. If I could solve all membership needs within D8, I'd be a happy camper for sure.