Steam has a “Go Offline…” options for pretty much that. Indeed it sucks that you have to do that before you go offline, but it sounds like a good idea with your setup and just switch to online occasionally to update.
Furthermore it depends heavily on the games, not on steam. Some steam games work without the steam client, though for some of those you have to fiddle around or execute different binary.
Steam has a “Go Offline…” options for pretty much that. Indeed it sucks that you have to do that before you go offline, but it sounds like a good idea with your setup and just switch to online occasionally to update.
Furthermore it depends heavily on the games, not on steam. Some steam games work without the steam client, though for some of those you have to fiddle around or execute different binary.