I am so sorry to hear about your kitty. I strongly second Stryker's suggestion about joining Tanya's CKD support group. I did, on his recommendation.
My Celia is 17.5 years old, in stage 4 CKD. Her numbers last month: BUN 104, creatinine 4.7, and phosphorus 7.3. Her potassium is also low, 3.5, and she also has anemia (HCT 20%). Back in January, the BUN was 97 and phosphorus 10, and the vet said it was time. But based on the suggestions I got from members of that support group, and the knowledge I gained from members as well as from the website, I told the vet that I wanted to start potassium supplements, a phosphorus binder, and an ESA if her hematocrit dropped below 20%. I also had to ask for anti-nausea meds, an appetite stimulant, an acid reducer, and pain meds for her arthritis.
Although Celia's now in her final few days, the help I got from that group has given us almost another 3 months together. In fact, we probably could go quite a while longer if Celia were a kitty that tolerated stress and treatment better. However, I can't do fluids at home, syringe-feed, give her pills, etc. because she gets so stressed that she will hide and run away from me for days after a single treatment that isn't even painful.
There are many members there whose kitties have been stable for a long time. even with high numbers. I would add a B complex and more vitamin B12 (but pills, not by injection). I hope you're using aluminum hydroxide (Amphogel) as a phosphorus binder?