Her
Creatnine is 5.3
BUN 62
phosphorous 13.4
Potassium 3.1
RBC 4.1
Hemoglobin 5.9
Wow. I am so sorry that you have been hit with this information. Hugs to you. I know what that feels like. That phosphorus is way too high and you want it in the 4-4.5 range if at all possible. You will need to either feed a renal food or to use an Aluminum Hydroxide phosphorus binder that is given with or mixed into every meal to bring that down. The phosphorus being that high will put an extra strain on her kidneys.
With a Creatinine of 5.3, it looks like you should start daily subcutaneous fluids if her heart is stable and able to handle fluids. I would ask the vet about this. Fluids can make the cat feel much better, ward off dehydration and bring the BUN down in many cases.
The potassium is a little low. Right now, with a 3.1 level, I believe you can handle this by adding in some Vetoquinol Renal-K powder to her food once or twice a day. I have good luck with the powder. It's chicken liver flavored and most of my cats (those who have low potassium) tolerate it well.
Also, the RBC and Hemoglobin are quite low. You should ask your vet about whether it's time to being Erythropoietin Stimulating Agents (ESAs) You may have read about these on Tanya's felinecrf.org site. I have very good results from these. But first, something you can do more immediately is to promptly begin supplementing with B-Complex vitamins. This can really help bring up the Hematocrit (you can read about it on felinecrf.org. I use Jarrow B-Right B-Complex Vitamins (many people on Tanya's support group use them) and split one capsule into ten size 4 gelatin capsules (using a stand made of flat pieces of Lego to hold the size 4 gelcaps while I fill them. Don't stress too much about how much gets in the gelcaps. There will be a little spillage. Just try to evenly fill the gelatin capsules with one capsule of the B-Complex.
Honestly, I strongly suggest joining the tanyackd support group on groups.io. There are a dedicated group of people there who are very knowledgeable and supportive. They can answer a lot of specific questions. Don't give up. There's so much you can do to improve Kit Kat's quality of life.
Also, did the vet confirm that the urinary tract infection is gone by repeating a urinalysis and even a culture if necessary? Just because symptoms are gone does not mean that the UTI is gone. I have had asymptomatic cats (doing absolutely nothing to indicate a UTI) that I took in for a routing urinalysis and many cocci (rod shaped bacteria) were found in their urine. I'm just suggesting this because a UTI can skew the kidney values in the bloodwork. If a kidney infection were involved, I would expect to see elevated WBC. Do you know what Kit Kat's white blood cell count was or any of the other white cells (neutrophils, etc.)?