Sean, just want to thank you for asking this question which has generated so much informative discussion. I, too, wondered at what BG would a kitty be considered "out of remission".
My cat Meeja was diagnosed in Aug '17 with a BG over 600, was hospitalized for several days and started on 1U Lantus twice daily. I was following my vet's advice on dosing (quickly raised to 2U, then back to 1.5U) and was not home testing but periodically taking her to vet for tests. Big mistake on my part. About a month after diagnosis, Meeja's BG went very low (40s) and I again followed my vet's advice to stop giving insulin "cold turkey". Meeja has been OTJ for 6 months now, but according to some of the responders, she would not necessarily be considered in remission. Like Rufus, Meeja's BG has periodic spikes but seems to come back down. In hindsight, I wish I had the information & knowledge contained in the responses to your question because I would have continued Meeja on insulin and tapered off the dose, as you are doing. I've thought about re-starting her on a small dose, but that idea really drives my anxiety level through the roof at this point in time. I feel sad to think I might have jeopardized her chances for long-term remission, and as Sandy noted, increased the potential for ketones/DKA. Sometimes it's just so difficult to know what to do.
Wishing you & Rufus continued success.