I somehow missed this thread before, but I have a bit of experience with this.
My wonderful cat, now deceased (at the age of 21. Damn. Just old enough to vote) went through much of what you describe. She was about 15 and got very thin. I noticed she drank and lot of water and the litter box needed to be changed more often and it was heavy! Her coat became dull and dandruffy.
She was tested and found to have diabetes, and was put on insulin, twice a day, with a very fine gauge needle. The insulin was cheap and lasted for months. The disposable needles cost a bit more but were not horribly expensive, IIRC.
Within 3 months, she was back to her normal, if svelte, self. Shiny coat, lots of energy. It was like she’d been revived or something. The vet switched her to a fish-bound protein type of insulin after a couple of years (sorry-- I forget the name. It had a "z - dash - something in its name) that lasted longer, and kept the insulin levels more constant.
So yes, it can be a way of life for these cats. I believe mine felt better when she received her injection. In fact, if I was late giving it to her, she’d come and meow in my face, then calm right down afterward.
She lived another 6 years on insulin, and died of a liver tumor at 21.