When they need something to chew on try the big raw marrow bones( shanks) from your butcher. Make sure they are long and good sized width, so they don't try to swallow them. They are great occupiers when you can't be with them.
Might want to try the Busy Buddy Squirrel Dude by Premier. Very tough to get the food out of with the little flaps inside the opening. I would get the Large or extra large size depending on your dogs size.
Very tough toy. And yes..our dogs do excel at those things they spend a lot of time doing. So perhaps a bit of trick training or obedience each day would help with mental stimulation. Trick training and Obed. games don't need to take long. 10-15 min. a day .
Dogs do get slab fractures from any weight bearing bones if they are aggressive chewers.
My Squirrel Dude toys have lasted through many dogs.