Need advice/help with my dog’s diet/behavior

Hello! As the title says, I need some help! Before we start, my dog’s name is Mei, and she is a 2 year old purebred Shih Tzu. Her current diet is comprised of shredded boiled chicken breast with boiled peas and sweet potatoes.

I took my dog to the vet a couple weeks ago because she wasn’t eating the food I made her (not the above recipe). I was told to use the balance.it website to help with future food making, and I have a couple of questions:

1. Is this website legit? I cannot tell if this is an AI generator trying to sell supplements.

2. I was concerned that one of the questions wasn’t about the breed. Isn’t that important for recipe building? If not, let me know.

3. Does the generated recipe measure for the whole day? I believe so, right?

4. The recipes says it has “15 deficiencies”. This is where the supplement product comes in. Is it safe for dogs? Why isn’t a recipe generated where all those deficiencies are checked off?

5. I give my dog Zesty Paws 8-in-1 multivitamins. Are these good for them? I’m guessing they don’t give all her daily nutrients.

6. I’ve been noticing she’s been slowing down more; not playing as much/at all; laying down more; not as social, etc. Is this due to her diet? Or is it due to her growing up and mellowing out? The meals I made for her before also lacked in nutrients, but she was still playful and energetic (it’s been about 2-3 months).

7. I am getting her spayed soon. She is a very friendly dog. Could the operation cause her personality to change? Is that a thing?

I believe that’s all I have for now. Sorry for the long text. Thank you in advance for y’all’s help!