If I understand you correctly, It sounds like you want an off the shelf business system (accounting etc) and customer CRUD application with billing that you can

If I understand you correctly, It sounds like you want an off the shelf business system (accounting etc) and customer CRUD application with billing that you can just wire into your unique functionality. I am not aware of any products that would meet those requirements specifically. There are a few products that might be connected using something like Zapier that would fit the bill, but nothing natively that does all this that I know of.