Hello and welcome. Love black and white kitties.

You have reached the best place you never wanted to be.
Did Felix have anything else going on with him at diagnosis? Infection, bad teeth, on any meds? You will be glad you are switching him to wet food. That's much better for cats than RD. Back in the day before my diabetic girl made me learn about nutrition, I worked part time at the local shelter. We had a room full of fatties on RD. Not a single cat lost weight.
If you are keen to start insulin, you can always pop into the local pharmacy and get a human meter. If nothing else, it can be used as a backup to the one you ordered. It is always good to have a backup meter. We have heard lots of amusing stories about meters being microwaved, dropped in fountains by tail swishing, etc.

If you tell us which country you are in, we might have suggestions for meters. Look for one that requires just a small blood drop. And most important, reasonable cost of test strips.
Due to the nature of the Lantus depot, it usually takes a few days for the depot to build and start seeing better numbers, but every once in a while we have a kitty that has a strong initial reaction to insulin so we suggest starting insulin when you can monitor more closely. We have a saying here, ECID, or each cat is different. Each cat needs however much insulin they need, and we have large cats on small doses and small cats on large doses. It's much easier to start on a lower dose of insulin and work your way up than finding yourself with an overdosed cat while you are still learning how to home test.
Speaking of which, make sure you have some high carb wet food stashed away. The fancy Feast gravy lovers are one such option as long as your cat can handle the wheat in gravy.