If I can make one observation about kale: if it is even slightly water-stressed on a regular basis, you will have tons of aphids that are impossible to control.
It grows well at my place if it gets direct morning sun only, rather than in very hot sun all day. This year I noticed lots of syrphid flies visiting my kale and they cleaned out the odd aphids I had. The syrphids are a family of hover flies that look similar to solitary bees. I have heard of parasitic wasps that eat aphids before but this is the first time I have seen aphid predators in action.
Before, I used mild solutions of soapy water to get rid of aphids but they always came back. Now, being more proactive with watering has kept them under control