I think a concern for the business is whether other people can help maintain the system. As such don’t go too custom and roll your own. Take things like nextcloud and see if you can fit the requirements by bolting on a few docker services. Keep it simple by using “appliances” where it makes sense (dedicated NAS?).
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!