This has probably been done to death on here but as experienced TKD people are, generally, kicking experts, I thought that you may be able to offer myself a little advice.
As some of you will be aware, I've been training in Kung Fu for a little while (~ 2 1/2 years). My leg flexibility when I first started was really poor - I could barely chamber my knee above waist height - and due to an operation I had on my right hip when I was little, I'm told that some of my muscles surrounding my right hip are shorter than my left.
2 1/2 years later and I can roundhouse, front kick and outside crescent on my left leg comfortably at head height - albeit, my technique isn't the best. With my right leg, the same is true but at around higher chest height - it might be worthwhile to note tar my right chamber is weak and I find it difficult to hold it above waist height, still. Side kicks are only effective for me with my left leg at lower chest height and with my right, bladder level - I can kick higher, but with considerably worse technique.
So, to improve technique, and build the strength required to execute reality high, powerful kicks I've been doing so e slow kicking in addition to my usual training for the past 6 months. The issue is I can only just kick above waist height really slowly with most kicks on my left, whilst my right, thigh height is about as high as I can go without compromising technique. I can feel the muscles in my left leg working overtime due to the pace of the kick, and I think with a lot of time and effort this will help get my kicks higher and higher, but with my right, I don't feel that I can even get the slow kick high enough to work my hip muscles enough to make much of a difference.
As the kicking experts that you all are

would anyone care to share what has worked for them - or their students - to achieve strong mid to high level kicking?
I feel that I'm putting in a hell of a lot of effort for little return.