How soft does a Kohlrabi get when cooked?
I spoke with the farm that sold me the kohlrabi; they have seen people make something akin to mashed rutabaga by boiling kohlrabi for ages and ages, so it gets about that soft. Note that mashed rutabaga is apparently still firmer than mashed potatoes, but obviously soft enough to mash into a pulp.

For a stir-fry, they suggested a match-stick shape rather than medallions; my second such stir-fry found them to have about the texture of halfway-cooked onions, soft enough not to crunch anymore but firm enough to hold their shape on a fork.