I admire your persistent efforts at keeping insect pests away from your kale plants. However, kale is in the brassica family, which includes broccoli, mustard and cabbage. I have included an article that lists many brassica insect pests and the damage shown in that article and the damage shown in your photos are very different. The pests in the article all create some kind of hole or “windowpane” effect, all affecting the leaves. None show damage to stems. While neither you or I may be able to figure out how, I think your problem is a critter getting to your plants. Stems that are gnawed straight across, as yours, are likely the work of rabbits or chipmunks. These little animals are particularly adept at sneaking under what we see as impenetrable barriers. Creating barriers to these four-legged pests requires a metal fence of some sort buried beneath the soil surface and extending high enough to discourage larger animals like rabbits. Frankly, I don’t know anything that will discourage a determined chipmunk. I know that’s not what you expected, but I believe an animal is causing your kale leaves to disappear. You might want to contact your local extension office so they can see exactly what it going on. I hope I’ve been helpful. https://lgpress.clemson.edu/publication/identification-of-common-insect-pests-of-brassica-crops/