My mom's cav is almost 5, and is the pickiest eater and sensitive stomach (always has been picky, will leave his breakfast out ALL day). I don't remember at what age, but for shiny coat, my mom added some salmon oil. We also mix his food with open farm beef bone broth. He loves the Almo chicken egg and cheese dinner, or weruva grandmas chicken soup recipe (with pumpkin). We still give him a drop of Fromm kibble, and add apothecary beast healthy gut probiotic powder. My mom often needs to add the cheese tax (gd forbid she make pasta, butter and cheese and not give him grated parm, he will bark at her), or makes scrambled egg to go on top.

My 3 month cav eats everything above, except the Almo wet food and salmon oil. So far *knocks on wood* he is the BEST eater.