Great job with catching that 39.
I agree with Julie. It takes 5 - 7 days for the insulin depot to form. That leaves about a week to look at how Thumper is responding to Lantus. That's insufficient time to evaluate the efficacy of Lantus. Given the similarity of Lantus and Lev, I would not expect to see much of a difference and, in fact, there would be another week of starting over with a new insulin, high numbers while the depot forms, etc. I would want to wait at least couple of months (not weeks) before determining if this insulin is a good choice for this cat.
Right now, Thumper is reacting to the drop in BG. It can take up to 72 hours for the bounce to clear.