It'd be tough to assess whether just pickiness (ie, sick of current food, just doesn't like it) or inappetance without trying new/novel foods to see if they then gobble it up.
I would suggest you try something like Sojos - this is what I use and I've also suggested it to several of my dog-sitting clients and they later profusely thank me bc their dogs are so in love with their food, it's crazy.
I add raw meat to my mix -- but you can just add cooked meat of your choice to your mix, to keep it within the diet guidelines you need.
https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1 https://smile.amazon.com/Sojos-Origi...sojos+dog+food