Hi Ellenellen! I know you would love for me to say to eliminate standing leg work, but I can't. Eliminating squats and lunges from a leg routine is the equivalent of eliminating crunches from an ab workout.
Unless you truly can not physically perform these exercises, weight bearing compound exercises, such as squats and lunges, are functional exercises that stress the muscles in ways that they are used in everyday life. Not only will this help prevent osteoporosis but it will help prevent injuries and postural problems brought on by everyday life.
It is always best when weight training, to include exercises that mimmick the body's natural way of movement. In a typical day we bend down to pick things up, walk up and down steps, sit and stand (now you know why I created sit n stands

)as we get out of cars, chairs, sofas etc. All these activities simulate squats and lunges and therefore recruit those muscle fibers. To condition the body in this manner promotes overall longevity and quality of life.
You have stated that your knees have gotten stronger in the last 2 years of weight training and that is absolutely why you need to continue. I know that you favor floorwork and I encourage you to continue with that too. If standing floorwork is not your favorite, then cut down to incorporating it once a week (one VERY FOCUSED time per week). If you ever feel twinges in your knees, try the following in this order:
1)slow down your reps
2)lighten your weight
3)do the exercise without any weight at all
Good Luck!