While I have no experience in the area, it is my understanding there is very little interaction between your computer and Steam, except under one condition and you can fix that.
Steam is required to validate your copy of the game as legitimate (if it’s not, then there are more problems). I have no idea how often, but that takes a few bytes and then is over. You do not need data from Steam, or Steam from you other than that.
The condition: by default the Steam cloud saves a copy of your game so it is portable, from one computer to another. I have it turned off since it does require bandwidth. Here is how to turn it off.
Open Steam and look for ‘Settings’. When you open that window, click the lefthand tab ‘Cloud’. There should be two checkboxes and the first one will prevent that cloud save. I deselect both boxes since I don’t want to save pics to the cloud, either.
That will limit Steam internet traffic to the minimum and should have no effect on the game.
The updates will not prevent the Data Rights Management that is inherent to Steam.
Good luck !