You may not have to give up two wheels
The Motorcycle Ergonomics site is a good start. But it is not perfect. Among other things it does not take into consideration the weight of the rider which compresses the seat-and that can change hip, knee, and other angles very subtly.
But please note that BMW motorcycles (other than the R12C's) have the distinctive European riding position where the foot pegs are behind the knee. Therefore one is loading up the knee and hip areas. I have the same problem. And I am doing more Yoga than ever before with an emphasis on hip and groin openers-it works.
So in order to decrease the stress on the knees, hips and back one can:
1: get a higher seat
2; Get the kits for lowering and moving forward the foot pegs, shifter , and brake pedals.
3.) employ barbacks and risers for the handlebars.
Each of the above will incrementally open up the knee angles and soften hips to knees. Unfortunately you will spend a lot of money, and you may not get the desired effect. Remember, adding a half inch here or there, really only softens the angles a few degrees. Been there-done that.
My next BMW will be an R12C. If I cannot find a clean used R12C, then I am off to a different brand in the 800-900cc cruiser style/midway controls type of bike. Unless you can perfect your riding position on current BMW, you are probably headed in the same direction.
My physical therapist who works with athletes, dancers, people like us and others put it like this: "Motorcycles are a bit like shoes. They should feel comfortable from the onset without modifications". He may be right.