Have you tried Tripett? It’s canned green tripe. Most dogs go nuts for it – you can mash it up in a little warm water and use it to make a gravy and coat the kibble.
Some brands with small kibbles that I can think of off the top of my head would be Nature’s Logic, Horizon Amicus, Wysong Epigen, Fromm, Addiction, EVO Small Bites, Merrick and Victor.
Something else I’d suggest is Abady. It’s a granular food that’s super calorie dense (800+ per cup) and very palatable. It’s not GF, but it only contains a small amount of rice and >94% of the protein is from meat.
You could also try looking up recipes for satin balls.
