Hi, I know how sad it is when you hope your kitty can do without help. I thought the exact same thing. But it seems to me your Lucas should stay on the zd dry for now since he is not reacting to it so much, and that way his immune system has a chance to recover and get strong.
On the prednisone, I wonder if your vet can give him a shot each month instead of the prednisone pills. That way it should be a little easier because it should mostly bypass his digestive system. My Curly needed a LOT of time to stabilize on both his food and his steroid shot. He needed some time to feel normal and to not be so inflamed and feeling sick.
I introduced the allergy pills while he was getting steroid shots. They contain the vitamin C that boosts his immune system too. Someone also mentioned giving Lucas L-lysine. Curly is at the adoption center and they are giving him L-Lysine along with his allergy pills and his special food.
I think your Lucas might have a chance to feel a lot better and to get stable if you ask your vet to do a once-a-month steroid shot, plus the zd dry food, and the allergy pills, plus L-lysine. My FIV group also recommends I think lactoferratin. I need to go back and see the exact description. Curly is not getting this now, but he would be even better if he was.
I know what you mean about accepting the situation. I don't know if your Lucas is FIV+ or not, but skin allergies and skin immune reactions are extremely common in FIV+ cats, and my experience with Curly was/is that I had to give him all the things to help him stabilize over at least three-six months. During that time I also gave him the allergy pills, hypoallergenic wipes, and kept him on food he did not react to. The vitamin C is in the allergy pills. I could have also given him the L-lysine and it would have been more helpful but I did not.
Like you, I wanted to see him get well and I wanted things to move a lot faster than they did- in my case especially because Curly is a foster boy and was in a foster room by himself. I needed him to get well even to be considered for adoption. I wanted him to catch the train for his adoption. I learned, frustratingly, that I had to just take my time and focus on baby steps to just clear up his allergies and itching and to get him on the allergy pills and let everything slowly get better. It drove me crazy because he had to be in his own room all this time.
It was also a huge help to get the steroid/prednisone shot instead of pills- why make the kitty eat pills which just causes more stress. Maybe a shot will do the trick. I would look at a regimen of slow, steady improvement and try to get the shot, still give him the zd dry, incorporate the allergy pills and L-lysine, so his immune system can improve to where he is strong overall. Then, 4-5 months from now, you can begin to introduce alternative foods and see if he can eat them. You will know right away after a day or two and if it doesn't work you can switch back fast and it will not be a horrible plunge like it is now.
I hope my experience with Curly over the last 10 months will save you some time. He is now in the adoption center, he gets occasional steroid shots, they feed him the allergy pills and l-lysine, and have found a dry turkey food that he does not react to. I NEVER thought I would see the day where he would be at this point. Good luck with your Lucas. He is lucky to have such a dedicated mom.