So being devil's advocate here, if you found a food that seemed to keep you healthy, your skin in good health, your digestive system stabilized, and good hair growth, why would you mix it up for yourself?
I personally, wouldn't change a thing.
And if you found that perfect food, by all means, don't change a thing! I totally understand your stance on not wanting to try something new. Your case, as you pointed out, is also unique in that way that you are dealing with liver issues AND hypothyroidism. If your dog's bloodwork is good, than by all means, please continue doing what you are doing. Why take the risk of getting his numbers off?
Unfortunately, over the past 40 years of my life, I have not been that lucky to have found that one "perfect" food. Therefore, I choose to feed a variety.
Here is an example, to try and explain to you why I do what I do. My sister in the 8th grade gave up eating red meat. Now (in her mid thirties), if she even has a small piece of red meat in her system, she gets severe stomach cramping and she gets very sick. This is because her body no longer has the proper bacteria in her system to digest the red meat.
The same thing happens to dogs. They get fed the same thing, year after year after year, they lose the bacteria in their stomach to digest the different proteins. It is proven science, that if you feed varied protein sources, the gut has LOTS more bacteria in it that can digest a larger variety of foods.
The plus to this, is that when I switch between the foods, because my dogs are used to this, they do not get a GI upset. So, if they DO have a GI upset, I know that it is not from the food, but from something else they got into as in the wildlife poo Dandie ate last week (too bad for me, they do not have a wildlife poo variety)
The same goes for if they have any changes in stamina, hair condition, etc. If there was a change, I would go for bloodwork. I get an annual CBC done on my girls, and I also do titers for my dogs instead of annual booster shots. So, I am on top of what my dogs "numbers" look like.
I hope that explains my side a bit more to you. And please know, I am not pushing my ideal on you, because I am well aware that this is not for you. You have health issues that you are trying to manage, and you need to do what you are comfortable with for your dogs.