Allergy friendly cognitive diet??

Hi everyone. I have a 9yo dog who was fed Nutro Ultra her whole life. Last summer she started displaying early dog dementia behavior. In August, I switched her to Purina Pro Plan Bright Mind. There has been a noticeable improvement in her behavior, but in November, she started throwing up much more frequently. Like at least once a week. She never seemed to feel bad and the vet was not concerned. In December, she started getting red patches on her belly and would lick them a lot. Vet did several tests and again was not concerned. Said it was likely a food allergy. The protein in her old food was a mix of chicken, salmon, and lamb. The new food is all chicken. I’ve transitioned her to the Purina Pro Plan Sensitive Skin and Stomach - Salmon and rice formula. The vomiting seems to have stopped and the red patches on her belly are almost gone, however the sleep/wake cycle issues are starting up again.

Does anyone know of a food comparable to PPP Bright Mind that does not use chicken as the main protein? Or does anyone have any experience with cognitive/neuro supplements?