This is about the variety idea with commercial diets.
I definitely don't think variety is a bad idea. But here is one thing I recommend about commercial food and variety: hold back some protein sources. If you do go the variety route (or even switch occasionally) in order to cover the nutritional bases better, you'll want to have at least one protein source up your sleeve in case a food allergy ever develops.
Food allergies develop after repeated exposure, and they are most often to a protein. Also, there are myriad possibilities for the other parts of commercial foods (potato, sweet potato, etc.). The challenge for an older dog who has eaten all kinds of commercial foods is to develop an elimination diet if needed. An elimination diet is a "novelty" diet -- limited ingredients that the dog has never eaten.
So I'd keep, say, buffalo and venison, or salmon and duck, or whatever, in reserve for that reason.
Even fish hasn't panned out too well as a novel elimination food because of all the dogs who live with cats and steal their food. Lamb, which used to be what vets recommended, long ago became much too common to be novel.
Just a Helpful Hint!