Hi Rikki and welcome-
I can appreciate your desire for science based medicine. To touch on a the points you hit on, no protein is not a factor in terms of correct growth for large and giant breeds. However, Dr. Rebecca Remillard ACVN states on her website petdiets.com, that growing puppies do not require a food with more than 25% protein.
Absolutely, by no means, should you feed your mastiff an adult maintenance diet. This is a tired old myth still thought to be true because it “worked” 30 years ago. Companies are now producing large breed puppy diets designed specificially for slow growth.
The safe upper limit for your calcium and phos ratio is 1.5:1. However, you also need to pay attention to the calcium to calorie ratio as well to keep the pup lean during growth. Overfeeding is a huge factor in devlopmental orthopedic disorders.
As for the choice of food….the two companies that have done the most research on large and giant breed puppy growth and nutrition are Hill’s and Purina. Selecting a large breed puppy diet from either of those companies would be fine. I do understand that some are opposed to using a food from either of those two companies. Dr. Tim’s Kinesis which you mentioned earlier, is safe for a growing giant breed, is formulated by a vet and a nutritionist and has been feed trialed. All qualities that are excellent to have in a food. Not to mention it comes in a 44lb bag on chewy.com. This would probably be my first choice for a budget friendly, good large breed puppy food, not from Hill’s or Purina.
Here is a good article to read that is posted on DFA here:
/best-dog-foods/best-large-breed-puppy-food/
and this is another from the persepctive of a veterinary nutritionist, that outlines DOD’s and large breed puppy nutrition fairly well:
https://msu.edu/~silvar/hips.htm
