I need a client portal to connect with max classic

Does anyone know of a software that can allow us to give our customers the ability to create a back end Customer account / business profile?

I found Memberium but it feels way to Ecommercy .

Our company is building an online platform that is selling advertising to other businesses and this application needs to feel like a highly secure back end portal that can communicate with us clearly and where the client can upload documents, images and answer general information needed to process their ad. This portal also needs to manager their monthly payments and I am hoping something like this exists that can integrate into Keap Max Classic. Thank you!