When it comes to trying to get a legit medical issue under control, I'm on board with sticking with the specially formulated prescription foods if that's what your vet recommends. If I couldn't find a food my dog liked on my own, I'd go back to the vet or a veterinary nutritionist and see if they could recommend or order something different, or suggest specific non-prescription foods that meet the dietary needs that will support his kidneys. I'd be very careful about just trying a non-prescription food without veterinary guidance thinking it's "higher quality," because something that is high quality and healthy for a dog without health issues could be actively dangerous for a dog with existing kidney problems.
If he were my dog, I'd at least try to stick out the prescription food for six months to see if you can get his blood tests back down to normal, then work on finding a maintenance food that will be appropriate for supporting his kidney health going forward.
I'm not a fan of grain-heavy, highly processed foods under normal circumstances, but sometimes it's medically necessary, short- or long-term. Veterinary diets exist for a reason.