As another thought, if you can, once I started blanketing Lacey a few years ago I REALLY noticed an increase in how much she moves around her pasture. It seems like the blanket helps keep her muscles and joints warmer so she's never really stiff. She does have stiffer days but not anything a few minutes of gentle movement can't work out. In Lacey's case, the colder she is, the more uncomfortable she is so if I keep her warmer she's MUCH more comfortable.
I blanket Miss Lacey as soon as daytime temperatures are hitting 50F with a sheet and move to heavier blankets with every 10 degree drop.
Of course, Del might hate being blanketed that much but you also might notice an immediate improvement, like I did.
Is there any way that you can get her moving (at a walk, maybe hand walking? Or ponying?) for about 30 minutes everyday? I've heard that daily movement is crucial for seniors (it's crucial for Lacey but in a different way - to keep her from killing me with her antics! Haha).
Maybe that's all stuff you're already doing. Lol!
She's looking great! I think her weight is perfect. Not skinny and not fat.
I love old horses. They're the best. :)