Regardless of the team size, I say simplify as much as you can so you can dedicate your resources customizing what makes their business special. You mentioned a PBX system and no infrastructure, this makes me think you talking about Customer Management. It sounds like you’re documenting as you go, fantastic. Maybe loop in a noob time to time to review the documentation or have a Q&A that reifies the docs. Best of luck!
Was the paper optimizing for multiple objectives? Sounds like this project is pursuing some features that haven’t been proven