Osmocote doesn't supply nutrients much past 4 months. From your description the Osmocote is probably pretty old. I don't think you need to cut back on fertlizer.
I agree - it is probably a time released fert. - some times you can just gently shake most of it out if it bothers you. I really doubt it will be an issue though. But, your pics reveal some really healthy new root growth. Looks like you are doing well by your cats!
Steve