Are you open to learning to home test his blood sugars? Testing at the vet can often produce much higher numbers, due to stress. Plus it's cheaper to test at home, and allows you to learn how to change his dose without the vet. And allows you to get his blood sugars to a better place. Pictures and videos of people home testing are here:
Hometesting Links and Tips
Vetsulin is not a good insulin for cats. Check out
guidelines published by the American Animal Hospital Assn for the treatment of diabetes. Prozinc and Lantus or Levemir or much better and longer lasting insulins. Vetsulin is only recommended for dogs. At Neptune's size of dose, I'd actually recommend Levemir, as Lantus has an acid base that can sting at higher doses.
IAA is a blood test, which goes to MSU, same place as the test for acromegaly. I'd not worry about it right now, as you do have a positive result for acromegaly. IAA positive can impact how we do dosing, but can be tested later. Do you know what the IGF-1 number was?
What food is Neptune eating now? A proper low carb diet is also key to proper treatment of diabetes.
she knows I’m limited on finances and never mentioned options except surgery or chemo, which I don’t want to put him through that. I have never heard of Cabergoline. I’ll have to see if she can order it.
I'm afraid your vet isn't up to date, not surprising though. There is no chemo option for acromegaly. It's a benign tumour of the pituitary, not cancer. The other options are surgery or radiation therapy, which my Neko had. Back in the day, there was no medical option like cabergoline. The vet doesn't order it in, you can get it compounded. If you pop over to our forum on Acromegaly, you'll see threads about cabergoline. Here's the first thread I posted about it 6 years ago, it is still a fairly new treatment:
Cabergoline as a possible treatment for acrocats: discussion
Don't worry about being overwhelmed, it's perfectly natural at first. We can help.

And yes, I have a thing for tuxies!