Basecamp Style Workspaces

I’m a freelancer. I want to be able to not just create assets for clients, but rather design a workspace for them that they can access to generate copy for whatever usecase based on the prompts I’ve included which they don’t have access to. I want to be able to control their experience a little bit (e.g. have a dashboard or dropdowns they can choose from). I want to have a brand knowledge base for their brand specifically which they could always update/modify and save. The prompts I’ve written on their behalf should be able to reference this knowledge base. Finally, I want to be able to charge them for access (subscription)