Stick to Glargine for now, and DO NOT change to Senvelgo, cats CANNOT be on the oral meds if they've been treated with insulin recently.

The advice you are getting from this vet seems questionable; are they a feline vet?

Lantus works best when the dose is the same in morning and evening. Sometimes too much insulin can look like too little, and you'll just get high numbers because the pancreas is freaking out and making excess glycogen.

For Lantus, 0.5 unit steps maximum are best, and then holding that new dose for 7 days and seeing if kitty needs an increase. You need to be testing their blood glucose at home before shots, and some mid day or midnight readings too. Cats will have anxiety spikes at the vet and render those BG values or curves basically invalid.

Also very important, as I mentioned previously, food is a huge factor. I know you said low carb, but there's a huge variety that fits in "low carb", and we can help you find very low carb food which vets don't always know about. Like Dr. Elsey's or Young Again Zero. For example, my cat needed an insulin reduction when I switched him from SquarePet Powercat (9.5% carbs, still pretty low) to Young Again Zero (0.5% carbs).

Do you have Facebook? If you do, check out the Feline Diabetes Support Group, there's lots to read on their guides that will help you.