Curious about this. I originally learned two knee strikes that I remember. One the basic hiza-geri, usually aimed around groin level, maybe to the abdomen, then a kind of roundhouse knee that ends up going to the inside and outside of the legs, like a close-in version of a low roundhouse. This is pretty much what I've always trained, as I can do them as I age, and they seem the most practical choices to me. With an actual partner/opponent I have awlays favored head and arm control and kind of kneeing from the side angle, rather than the plum clinch type use.
However, recently I've been watching instructionals recently to pick up the Muay Thai style knees and clinching for fun on the heavy bag, I'm wondering how people incorporate knee strikes into their curriculum, do you use Muay Thai style attacks coming off of the plum clinch, as well as that kind of stirking, do you do something else, or do you do a combation of the two with your students?