If aloof is a requirement then this is NOT the breed but one worth considering IMO would be the Australian Stumpy Tail Cattle Dog (yes they are different than the Australian Cattle Dog).
My experience is that they are smart enough to be quick and easy to train but not so smart that they look for ways of getting in trouble. They are extremely loyal so you will never lose them off leash (only time I thought I lost mine was when he was sitting directly behind me just out of my line of site).
He is easy to bring into the office and other public places because he is shy with strangers so is never going to jump all over them or be a pest but is friendly if people stop to visit with him.
Mine might be the exception to the rule but he has never shown any inclination to chase horses, or other small animals. The only challenge was that it took him a bit to figure out the acceptable distance to follow on a trail ride because he would stay so close that if I did a down transition and he wasn’t paying attention he would literally run into the horse’s hind leg (thankfully I have patient horses).
They are a little smaller than specified (30 - 55lbs) however I have found that a handy size because my guy at about 35lbs is big enough to head out for any length run or hike but is small enough to pick up and move around if needed.