Welcome Margie and Jackson!
I am pretty new here too, but your post title caught my eye, as Peacock also came to us already with diabetes. She was a rescue with a bunch of issues, and the shelter did not discover her diabetes until a couple of months after they got her, then she was treated for diabetes for 6 months, but not really regulated, before we brought her home. We have had a really steep learning curve trying to get to know a new cat and trying to figure out how to manage the condition at the same time. This site has been hugely helpful!
Have you been able to do at-home blood glucose testing? Our cat has been very responsive to dietary changes. She was on a diet of both canned and dry Purina DM at the shelter, and based on what I read I wanted to cut the dry food out asap. It made a huge difference in her numbers, enough that the insulin dose she was on needed to be significantly reduced. We also did a trial of Tiki canned food once her numbers were improving, but her BG levels shot back up until I stopped the Tiki. It was odd, since the Tiki nutritional profile seems perfect for a diabetic cat, but it did not agree with Peacock. She gobbled it up, then she spiked. Her numbers have been much smoother on Fancy Feast Classic Pate. She still has canned DM sometimes, when the shelter gives us some, but I still feed at least half FF when she gets DM. I am pretty strict about food quantity, since I am trying to help Peacock lose a little weight. When she arrived at the shelter she was underweight like Jackson, but she went from 8 to 12 lbs in the time she was there, and now she has a bit of a bulge. She is getting closer to 11 lbs now on her at-home food plan.
Best wishes for you both in managing the condition