High value treats really do get results. Glad you had this breakthrough! You just have to set the treat bar high sometimes.
Poo eating is one of those unwanted but common traits... best preventation is to get it up right away so they don;t get tempted. Some pups seem to grow out of this too. I have cat poo eaters; dog poo doesn;t interest them at all. And Jaspar used to love the litter box but now never eats it. So this isn't necessarily a permanent problem.
PS DON'T free feed. With breeds like cockers and cavaliers you'll have very obese dogs. Poo eating isn;t connected to hunger; some dogs just like it. rarely it is linked to not enough protein in the food but some dogs just like poo. You can google it and find lots of advice/info. But best approach is to have it be your duty not to make it available by getting it up. It can be very hard to train a dog not to eat it. As they get older you can work on the 'leave it' command though.