It’s all trial and error. It sounds like you are finding out what works best for your dog.
If you are going to rotate, I would stay within the same brand for best results, in your case, Firstmate/Kasik.
But I would finish one bag first, then start the next, rather than have 2 bags open at the same time. Kibble goes stale fast.
For example, my local pet supply store had a good deal on Zignature catfish, so I have a couple of large bags to go through, after that it will be the whitefish (our favorite), next I will try the trout/salmon.
One of my dogs (allergies) does best on fish formulas, my other dog eats anything. So, fish it is!
But I add stuff to the kibble anyway.
