Crone, I wonder if you should go look at some of the local arenas? I like Laura’s, which is sandy loam and maybe some rubber at one time. You could ask her. She does take care to groom it a lot tho. I also like the footing at Oakhaven, which is more of an angular sand plus some rubberized stuff. I am sure Senja or Debbie would be glad to have you come visit. Those are the two I know about, and I like them both.
Be careful of the local guys. I think they do a lot of western, who tend to have their footing deeper and shiftier than I prefer.
I think a lot of it depends on how much grooming you want to do. I love the way sandy loam added to sand fluffs up as well as gives some purchase, but it does tend to pack.
ETA: I just re-read about the sprinklers. If you can keep it watered, I LOVE the Oakhaven outdoor when it is watered. Go visit!