Aussies will work depending on their pedigree. I’ve got a friend who breeds ASCA registered Aussies. The ASCA differs greatly from AKC registered as its not about looks, ASCA wants dogs that will work. There’s a breed specific to California called the McNab. A lot like a border but a little tougher for cattle work but I think they still work well on sheep. Kelpies have impressed me and I would like to have one someday. I’ve only read about this breed and I don’t know how easy they’d be to find in the US but they’re called Koolies, they’re from Australia. Smooth and rough collies will work depending on their lines. The New Zealanders have a dog called the Huntaway, but again that’s just a breed I’ve read about. I don’t know how an Idaho Shag would do on sheep but they’re good cattle dogs, same goes for hanging trees.
Bella is 3/4 border and 1/4 Aussie, she stalks like a border and does well getting around but she’s got some bite to back up the eye. Helps with obstinate ewes who think they don’t need to do as they’re instructed.