I will be 69 next weekend so I am younger than you are, but I will write something anyway. About twelve years ago, after a 600 km brevet that crossed the Blue Ridge Parkway three times, I decided that my click-in pedals were responsible for a lot of my knee pain. I switched to flat pedals and haven’t looked back. A few years ago, I ordered some 165 mm cranks to replace 175s on my touring bike because I sometimes had pedal strike when riding on trails. Since then knee pain has not been an issue. This year I was able to ride from my home in North Carolina to the Pacific Coast — took the train home — and then to the Atlantic and back. I had very little knee pain and that little bit was only transitory.
Click-in pedals have some advantages, but flat pedals allow my feet to relocate slightly any time they feel like it. I also notice that my right foot rides further forward on the pedal than my left and that is points out just a little bit. As for the crank length, I wouldn’t hesitate to go to 160 mm. I couldn’t find shorter cranks locally and ended up ordering directly from Sugino in Japan; that was pretty easy.