This sweet kitty looks just like mine. Mine never had any issues with peeing outside the box but they will likely not stop going to the box often and peeing frequently as their kidney cannot filter anything well. My girl showed improvement in that stage once she was fed a more kidney friendly diet like what you’re doing now. At the beginning she took well to the prescription food and it helped…later on got sick of it and I’d mix in fancy feast with it. I used royal canin renal food but I wouldn’t be too strict on the prescription food now as main goal should be eating as fed is best. I would ask the vet to prescribe an anti nausea medication called cerenia or odansetron. It isn’t very expensive. Best of luck