I would totally recommend looking at grooming gloves, if you are able to get your palms or the inside of your fingers against the horse at all, they should work really well. I have a weak wrist on my dominant side, so I find grooming and currying fatiguing, though probably not nearly as much as you do. I have the hands on branded gloves.
There are body brushes out there with 3 piece, jointed backs and a strap. Depending on your grip strength, those might accommodate your hands/fingers in a more “curled” position.
Another thought would be the Oster grooming tools with the handles. They definitely make 2 styles of curry (large tooth “massage” curry and a softer, smaller toothed one) and some type of body brush which are circular with a solid handle. You can buy the curries as singles, but might only be able to get the body brush in a kit? Link below to show what I’m referring to. They do have some soft touch silicone for grip, but unfortunately they are kind of heavy. I believe Weaver makes some similarly shaped brushes, but I haven’t used those.
https://www.statelinetack.com/item/oster-ecs-7-piece-grooming-kit/SLT054771/
My one other thought is if money is no object, you could see if there’s someone out there who could make some custom-handled brushes for you. I’m picturing body brushes with a handle that comes out and away from the brush backing, and is an easy diameter for you to grip.
If I think of anything else, I will add to this