What are the best tools for managing small clients’ IT systems?

I'm doing part-time IT support for a few small clients, managing around 5 to 20 workstations each. I'm on the lookout for better tools to help keep their IT infrastructure organized. I need something affordable or open-source, ideal for a small-scale side business. I'm looking to track details like which workstation belongs to whom, phone numbers at each desk, what OS and Office versions are in use, and connections to printers and network devices. It'd also be great to link my own ToDos and notes regarding what I've done for each client and when. Currently, I'm using Obsidian, which is decent but not really meant for this purpose. Active Directory isn't a good fit since I usually work remotely and don't want to remain logged into their network all the time.