there are so many opinions on dog food. One good place to look, even if it's just to compare ingredients is
www.dogfoodanalysis.com - most yorkie owners avoid 6 star foods because of high levels of protein in the food. The 4 & 5 star foods would be good choices
There is also a lot of info on home cooking for your yorkie here on yt, including recipes.
Yes, you definitely need something better than Purina