Sounds like my angel Easy. When she was 10 yrs, she lost weight. Could see her ribs. My old vet thought Easy had Kidney problem. The vet did urine culture, but came out neg. Did blood pressure checked up. We did everything like you did. Changed the food, still she didn't want to eat. She was picky eater all the time. I met Dr. Cuoto at Dewey, and told him about her problem. Sent him her CBC and urine analysis.
Found out, she was getting old and being a picky eater. She lived 4more years after this episode. She went the bridge with Osteo not kidney at 14 years old.
Hope Snow is just being a picky eater, not kidney problem. A big hug to you and Snow
Ps: Easy had only 14 teeth when we got her at 8 years old.