I don't know how many horses you feed but, for me the best solution to the wet feed was a new cement mixer with the tub that will mix 100 lbs of feed with the beet pulp. You can soak the beet pulp overnight if you want, just cover it .
Next morning add the remaining ration of grain let it mix then pull down on handle and dump it into your buckets. Ours cost $250.00 but it was well worth the cost we have had it for 3 yrs now it works great for any kind of mixing you do.
I've taken spoons, ladels, etc., out to the barn, bought special big huge spoons just for stirring feed, and they just dissapear, but the metal sweat scraper that I have had for about 20 years still remains...LOL
Too funny, I had the same problem, my arm was so sore from mixing grain glob that I developed Tennis elbow. Had to go to Doctor have horrible shot and now wear brace on arm for tendons. HERE is the solution
Buy a cordless drill and a cement/paint mixer attachment,,,, Let the drill do your work. I use the rechargable batteries. I mix sweet feed, pellets, bran. wet beet pulp etc, mix it all up dry then add the beet pulb and mix again. MY arm is very happy.