rugwithlegs

I worked on a few drills for giving the Uke a better sense of when Nage was under their center. Uke doesn't have to sell it so much as give you feedback when your position is correct. I try to keep the arm power minimal when working on hip placement and precision.

Being a taller man, Koshinage might not be ideal for you. Check out the basic Shodokan 17 kata. Tomiki Sensei was 6" tall, and I see his basics as being more geared toward tall people.

Judo has Koshinage variations for height discrepancies, I believe Hane Goshi and Harai Goshi serve this function.

In Budo Renshu, there is a drawing of the Founder doing a Koshinage by ducking under Uke's arm and throwing belly to belly. Some people call any technique where Nage displaces Uke's center with the torso a Koshinage, like a Taiji Shoulder stroke or body blow. There is a great deal of variety out there for defining Koshinage.

In terms of a Nikyo Koshinage or a kaitenage Koshinage, that's a little tricky. Usually those techniques start with breaking Uke down, so harder to get even lower underneath. Maybe a henka where Uke bounces up and you float him into the throw.

Sore knees - I found I could keep my legs further apart making squatting easier if I learned to shift my weight well.

I would be curious to see what you have been working on. Good luck.