Start with long legged no stirrup work, walk trot and maybe canter. It’ll train your balance
I don’t have good knees either (nothing bad, like surgery or real injuries, just they hurt a lot sometimes) but for me having longer stirrups makes them feel better. Also, I think that longer stirrups helps me balance better! Personal opinion though!
I have a bad knee. I have had one surgery on it about three years ago and I just had another one. Keeping my stirrups shorter always hurt my knee and made me very uncomfortable. Making them longer made me feel unstable. So I started to work on my balance. I really focused on having good posture at all times, I did little things to help my balance improve, and I started to ride bareback more. Riding bareback really helped bc it has your whole body sit the way it’s supposed to sit. It really helps with balance too. If u can balance perfectly while riding bareback, then riding in a saddle should be very easy. I say u start to ride bareback more and get more of a feel as to how your horse moves. It will help for when u make your stirrups longer and give u more balance confidence. I hope this helps!
You can always try things. That’s a part of the experience but I’d you feel too uncomfortable, remember it’s up to you and your horse. If you feel you gonna get hurt then don’t take the comment so much to heart.