I've been reading with interest as this goes on. The only area I have experience in is the exercise and I have to agree, the one hour in the morning and half hour at night may not be enough. It sounds like it is on-leash too? So even more, not enough. My 10 month old Lab is doing more, 50 to 60 minutes AM and PM, partly or entirely off leash. An adult dog could do a lot more than 1.5 hours a day and off-leash burns up way more energy.
Just think, a GSP is bred to hunt ALL DAY. They are a high energy breed and need lots of exercise. There can be a problem with giving more exercise to dogs like that, they just get more and more fit. Hard for we feeble humans to keep up, unless we are into marathons. Mental stimulation can really help fill in with something else for the dog to do. In a hunting home the dog would be working on what might be extreme obedience for us, absolute rock steady stays till bird is flushed, directions for finding the bird, precise handing off. I agree with Liisa that a combination of more exercise mixed with mental stimulation is certainly worth a try for this dog.