You mentioned a Kitchenaid stand mixer attachment. Kitchenaid used to make a shredder/slicer attachment that's very good for this usage. It had a wide hopper that could hold a quarter of a cabbage at once.
Unfortunately, the newer edition of this attachment has a smaller hopper and isn't suitable for cabbage, but the older style is still available from third party sellers.
Here's one example.
I'm good with a knife, and I sometimes use this for a single head of cabbage. It's definitely faster than hand slicing for anything more than two or three heads, even with setup and cleaning time. It also produces a very consistent cut.
It's definitely still going to require cutting up the cabbage heads first, and won't be as fast as some of the more expensive options, but it's an affordable alternative to the professional grade options.