I have had good luck with both Devil’s Claw and MSM. It seems to depend on the horse. I’ve found with MSM, I need to feed at least 20g a day to get a real benefit.
With my arthritic horse, I found riding him around five days a week for about half an hour at a time to work best. It’s seemed to be good for keeping him loose without making him too sore.
If it were me, I would stop jumping her, jumping is pretty hard on their joints. On days when he’s really sore, I would give him a Previcox at night.
For my horses, I have found the back on track products help amen slate.
Avoiding tight turns and on even footing should also help a lot.
Good luck. I hope you find something that works well for her. Take care.