I recently lost one of my fur babies with kidney issues so I've done a lot of research on food.
I will preface this with a couple of things. Food research will drive you INSANE. There is so much information (and misinformation) out there.
Vets aren't always the unbiased source of information you would hope they were. This is NOTHING against Vets who, for the most part, probably got into their field because of the desire to help animals. It's just that the education system is set up and paid for in many ways by commercial food companies that give them minimal training on nutrition and teach them to rely on prescription pet foods. Yes, I know some people consider this tinfoil hat territory, but there are a lot surprising facts on this.
Many of the ingredients in the expensive prescription foods are the same/worse than other sources.
That being said, after making myself insane, I realized that the best I could do was research, talk to my vet and come up with a plan I was comfortable with.
My google search started with "prescription feline kidney diet alternatives" and never seemed to end
In the end, if you're interested in alternatives to the food itself it's a journey you'll have to take to figure out what you're comfortable with and what makes sense to you.
Just a couple of quick notes on the off chance you decide to go this route. From what I learned, one of the most important things with kidney health is keeping the cat hydrated. This means no dry food since that actually dehydrates them more. Cats tend to have a very low thirst drive, the best way to get water into them is through their food.
If you decide to make your own food, run from any site that doesn't talk about things like taurine and other supplements that may need to be added.
Best of luck with whatever road you decide to take. Having sick pets is never easy.