Maggie is a sheltie, around 8-10 years old. When I adopted her from a shelter last May, I knew she had some health challenges including thyroid, allergies, and joint. We've adjusted her thyroid meds after doing a five-point panel, we've resolved the joint issues with Recovery and CBD oil, but the skin issues that are supposedly from allergies continue to be a problem.
When she arrived at the shelter, her ears were so infected it looked like they were plugged with "peanut butter" (in the words of her worker!), both eyes were infected, her fur was a mess of mats, and her skin was covered with sores. They put her on the Hills hypoallergenic diet for the month she was there; my vet (holistic - conventional plus certified in chinese medicine and acupuncture) recommended a Canadian made food by Rayne's. The kangaroo one seemed to help, though she continued to get sores on her belly.
Raynes is $60 for a 3kg (6 lb) bag, so I tried to reduce costs by switching to Zignature kangaroo instead ($30 for a 3kg bag). I did two weeks of half and half, and then two weeks of just Zignature. The sores on her belly all but cleared up, but now her back is a MESS - losing her undercoat, large red bald patches under her guard hairs, and sores that pop up overnight. So back to the Raynes, I guess.
My vet doesn't believe allergy tests are helpful, and we know she does better on grain-free kangaroo than on any other proteins. It is also likely she has sensitivities or allergies to environmental things as she gets more flares after hikes in grasses than hikes on duff trails.
But I'm also thinking her problem may be a systemic yeast infection. So I'm looking at probiotics and what else? My vet has also mentioned apoquel (assuming it is all allergy related), but that is well over $100 per month. I'm already using the whole of my Old Age Pension for animal expenses for my two animals (I have a cat in kidney failure also), not counting vet bills - just food, meds and supplements and monthly grooming for Maggie - so it is a good thing I also have a private pension from work. Things are tight and money IS an issue, but so is my dog's wellbeing. Clearly homecooking with kangaroo is not an option.

So, what has worked for you? What is your experience with apoquel or other meds? What have you used for yeast infections? What probiotics do you recommend? Have you done allergy tests and was it useful? Help!
Edited to add: I forgot to mention she is on benedryl - one a day, but I just increased it to two a day with some improvement to her back). And she isn't scratching a whole lot - maybe once or twice a day I catch her scratching or nibbling herself.