I apologize if this is a topic that has already been addressed, but I've been receiving a ton of conflicting information and would like to hear your thoughts - what is the most nutritious diet for Lady Gouldian Finches? Specifically, is a pelleted diet superior to a seed-based diet, or vise-versa? Or are there additional alternatives that I've not come across?
Allow me to explain. I've been keeping Goulds for two years. I had done my research and thought I'd obtained a sound pair, but after a few days I noticed my male couldn't stay on his perch at night and would twirl about helplessly in the dark even with a night light, and my female's one eye was perpetually shut despite trips to the veterinarian and numerous remedies. For these reasons, I firmly decided against breeding them, but I started them out on a millet-based diet supplemented with egg shells/cuttlebone and adding synthetic vitamins to their water (which is filtered using a reverse-osmosis filtration system). I also provided a full-spectrum avian bulb on a 12-hour cycle. They seemed to do well despite their disabilities, and were active, singing, and the like.
About a year ago, I switched them to a pellet-based diet with some spray millet as a treat. They molted for the first time since I'd had them and my male's issues seemed to lessen considerably. I continued to supplement vitamins. A few months ago, I converted them to Harrison's organic high-potency super fine pellets. Per the recommendations of the manufacturer, I stopped supplementing synthetic vitamins and millet altogether, continued the 12-hour full-spectrum lighting and eggshells.
A month ago, my male started to become less active and would spend most of his time sitting on a perch fluffed up. Over a week ago, I woke up and found him deceased on the bottom of the cage. After waffling back and forth with myself over if I wanted to seriously pursue the hobby of breeding these little guys and feeling guilty about my hen now being "widowed" and lonely, I purchased a new young male this weekend (who is in a quarantine cage as a precaution). The breeder advised me that it's possible my deceased male passed away due to malnutrition from the pellet-based diet, and that Goulds should be fed a predominantly millet-based diet. This is contrary to what my avian veterinarian has advised, but I've seen it repeated on various websites and in my little Gouldian care book.
Unfortunately, my hen passed away this morning - same symptoms: fluffed out, lethargic, and on the bottom of her cage, but she was eating up until the end. I'm trying my best to keep my other birds (my new male and my beloved Eclectus hen) contained away from anything that had contact with my deceased finches just in case it's something contagious, but now I'm a bit paranoid: did they die from malnutrition? What should I be feeding my new male and my eventual new hen so that they are happy and healthy, and can perhaps one day breed?
Thanks for reading my wall of text, and your tips are appreciated.

Regards.