Hello and welcome. Sounds like it's been a long journey so far. Sorry to hear the Bexacat didn't work out.
When my long haired girl Neko was diagnosed with diabetes, she used to try to raid my other cat's food all the time. He just after Neko's diabetes diagnosis, after got diagnosed with crystals. So I had a problem with her needing low carb wet food, and him supposedly needing the high carb food for his crystals. I talked to his vet about what food I could food both. The vet suggested raw food, so I switched to tgat. I found out later that what would also work is any low carb wet food that is lower in phosphorus, with plenty of water added. See this web page.
Cat Urinary Tract Diseases: Cystitis, Urethral Obstruction, Urinary Tract Infection. The website is written by a vet specializing in feline nutrition.
You absolutely do not need to feed vet food. Besides being pricey, the ingredients are not the greatest and they are very often too high in carbs for a diabetic cat. This list of foods for diabetes and CKD includes lower carb and low phosphorus and should work for you too.
CKD food Chart There are a couple options on that list that are novel protein (lamb, rabbit, pork or whatever meat she hasn't eaten before), that can also help with the IBD.
Prozinc is a good insulin for cats. If feeding a low carb wet food diet, you can start with 0.5 units. The glycobalance and crystals food are high carb and cats on high carb food we suggest 1.0 units.
If she has been off Bexacat since January and in higher numbers, you really do need to start her on insulin. There is some information in this post that will help you getting ready for low numbers. Happy to hear you have started home testing.
New? How You Can Help Us Help You! Home testing and having high carb food around to counter any lower numbers are how you can keep her safe.