Hello and welcome!
I'm sorry that the diet change alone wasn't enough. It's possible that the addition of some insulin support will help put him on the path to eventually becoming diet-controlled, only time will tell.
Regarding testing: we are big advocates of home-testing here! Your vet may prefer so-called "pet meters" to do the testing, and they are great meters, but we've found that BG meters made for human diabetics work just as well for managing FD, and are much cheaper to run over time.
Speaking of cheaper, lantus is a great insulin for cats, but there are cheaper ways to obtain it than a $360 vial. We find that the pens are easier to use and have amounts that are more suitable for cats. With the vial, you may find that you don't use it all before it starts to lose effectiveness. Properly cared for (refrigerated, not frozen, not shaken, etc.), it should last for more than the 28 days listed (that's just what the company paid for a study on). The pens can last for months properly cared for, so presumably the vials will too, the main issue is that the vials hold a lot of insulin and cats' doses tend to be tiny, so it's hard to go through them in a reasonable time.
Welcome again, it is all very overwhelming at first, but we're here to help in any way we can!