Hello & welcome,
Firstly. There are plenty of vets out there. If your vet has let you down before & you have little confidence in their abilities then I would be looking to change. Do some homework, read up on profiles of practicing vets, look at fees, consider what is important to you & your dog. Be cautious of those that push prescription diets ! If vet changed then maybe get a full MOT done, discuss history, concerns & any tests if vet & you feel they are needed.
I have reservations on giving feeding advice if your dog has dog has potential health issues. You indicate dog is overweight, lazy & unhappy with low food appetite.
Despite you going into some detail, your post didn't paint a clear picture of feeding. You make more than one reference to biscuits.......Your post seems to document a well intended but hit and miss approach. You appear to have fed kibble, cooked chicken a non disclosed source of meat, biscuits, chum & Naturo.
The only food item you indicate the dog seems to consistently eats is chicken. Keeping chicken in the diet may be a good idea, however, the dog has been lacking energy. You should not rule out the possibility that the dog has an appetite for a protein source that may not best suit. The dog rejecting all but cooked chicken may be down to it being more texture & flavour appealing than other bowl content.
Maybe try the dog on other single protein sources up to 7-10 days at a time, monitor appetite, stool output, condition & energy etc. The reason for doing this is to see if the dog perhaps gets on better with certain single protein sources better than others. You obviously need to decide what food type you want to use for your trials..If it were my dog I would stick it on a decent single protein source whether home cooked, raw or other food type agreeable to you & dog. Complete foods will have more variables and make it harder to know what works and what doesn't. That said I would add a simple source of veg if feeding home cooked. I am not a home cooked feeder & others on here far better able to advise on this if you go down that route long term !
Importantly I would want to have better idea of my dogs health before embarking on weeks or months of trial and error changes to feeding. My priorities would be....Get a good vet, rule out or manage medical issues, sort a fuss free diet that works, leave out the biscuit(s), do not overfeed (or let others do so) & ensure plenty of fresh water encouraged.
It goes against some advice but I frequently hear that most dogs seem to do ok on tins of wet Chappie. Not a high scorer on here but arguably suitable for dogs prior fussy &/or on back of recovery. Worth a try as stop gap if dog wont eat !