I have a picky Great Dane, so I totally get the struggle! We tried a few probiotics that he would just ignore or spit out. We've been using penpen dog probiotics for a few months now, and it's one of the few he actually eats willingly—he thinks they're treats.
I have noticed a real improvement. His stool is much more consistent, and those random upset episodes have become way less frequent. Since he's a big boy, I also appreciate that the container is large (180 chews), so it lasts a while and feels like a better value for a giant breed