I wanted something similar from a remote company I was working for. They were pretty good about fulfilling requests, but when I asked for a good kvm switch they said they had trouble in the past and instead recommended a usb hub that can toggle between machines. Then connect both machines to the same monitor and toggle the input. Not ideal, but low cost and functional. Might not suit your needs (would be annoying if you have to frequently toggle back and forth), but if you’re just trying to share your desk space between a work machine and personal, and the monitor input is easy to toggle, it’s worth considering.
I have used inkscape for this purpose and it can be effective. The display units can be changed in the preferences (inch, cm, etc) and basic shape dimensions input directly. You can input absolute position for shapes and nodes, but I didn’t notice an easy way for relative position. They also have a path effect called “Measure Segments” for that functionality. FWIW, I later preferred blender, but I might try a dedicated tool like LibreCAD mentioned in another comment. They seem to have decent documentation and a wiki.