Usually, when I see these threads I think, "dude, just start a new game", but I'm find myself in the middle of an interesting game that I'd like to see to the end and that might take a week or two more.  Do I need to disable Steam updates to prevent the saved being broken?

Nope. We generally try to avoid this, and post-release, it's been only on the odd occasion that it's happened (generally when we made drastic changes to the tech tree).