Most of the major feed companies have feeds specifically made for cushings and Insulin resistant horses. But as read above carbohydrates are a big factor. Which are most of your grains.
I have a 20 yr old pony that is on this diet and we have eliminated all RX medications[it works for her,yours may be different]
Beet pulp with out molassases-6 lbs total for the day,soaked weight
Complete advantage-Purina brand feed
Spillers Happy Hoof- Low carb. bagged haylage-theres carbs. in hay too.
MSM

You don't mention an age but they shouldn't get joint supplements. GLUCO[anything] Gluco =Sugars. Also now that spring is here be cautious about turnout time on grass.

As for the cinnamon, I just wouldn't, horses don't have access to it naturally, so there is no telling what damage it might do.
Chromium is a thought although new studies have shown it to be in-effective as well.