Well, I'm not a vet, but that's a pretty good blood test result for your ageing dog. Everything is within normal range. That blood work doesn't explain the mucous in the stool, it's generally an infection or other.
I still wonder if the dog is suffering from chronic intolerance due to food. If you look at the ingredients in the Acana Heritage, that's alot of ingredients first of all, and whole chicken is the 3rd ingredient. Chicken meal being primary, could simply be the beaks and arseholes.
https://www.dyrecenter.dk/images/Aca...ormula_1-p.jpg
Would you be willing to try a raw diet for a month? The concept behind this is removing everything from the dogs diet - and your dog will be sugar, carb, wheat, etc free and is the fastest way to rule out allergy or intolerance. You'll need to start eliminating the possibles.
When all else fails, go with the bottom up approach.