@bayhorse A couple thoughts.. You can also simulate riding by sitting on one of those big therapy balls (or an air filled cushion version of that on a chair) and if you rock back and forth as if you were on a horse, you would be working your core. Your feet stay on the floor.

For exercises, have you tried the stretchy bands that physical therapists have or elastic bands? Those provide resistance without adding weight.

My mom has severe osteoporosis and it didn't take any effort for a bone to break in her foot causing a fall that broke her pelvis, and a few years later, a compression fracture of the spine. Those are long recoveries when that happens, so you need to be so careful to prevent it.

Another thought... walking in a warm water pool for resistance against the water if you can safely navigate a pool and locker room.

I had an old horse who tripped a lot. He had been a free horse and given away because of this problem. He did improve with better hoof trimming, but got worse when he was older. A few times when older, he fell on both his his front knees when I was on his back, and somehow, I managed not to fall off with him dumping me forward over his head. I really learned to ride balanced because I never knew when he was going to loose his balance. I did have to retire him, and I got a younger horse that I ride. The old one is gone now and is the one on my avatar picture right now.