We rotate between the yellow, purple and red bags (forget the names). The dogs favorite seem to be yellow.
We get it from Chewy.com. It’s the same price as my local dog kennel that carries it.
(2) Blue Buffalo has been growing extremely quickly over the past decade. The company went IPO in 2015 IIRC. I highly suspect the cost cutting on ingredients has already begun.
(3) Blue Buffalo already sells lines which contain "grains and other irritants". I tried giving my dog "Wilderness" (the highest protein content with the least grain/filler) but it was too rich for him. For a while, I settled on a Blue Buffalo Life Protection which was cut with oatmeal, a grain.
I've used this site and their deep analyses to understand what is optimal (according to them): https://www.dogfoodadvisor.com/dog-food-reviews/natures-doma.... They also have a newsletter that emails whenever a recall of dog food/snacks is initiated.
Both of my dogs (smallish breeds) have been able to switch between with minimal effort to transition all that slowly, and their weight has been stable/healthy despite not nearly enough physical activity.