As long as they get their insulin on schedule, there’s really no suffering at all. Even the injection can be done painlessly through the scruff of the neck. All you’re doing is picking up their pancreas’ slack and providing insulin when it falls short.

There’s a little discomfort whenever you need to do a glucose curve/check, but it’s not too bad. You need a glucose monitor and test strips (not trivially inexpensive but not too expensive as a mostly one-time/rare cost). You use a lancet to prick their ear and draw a tiny drop of blood to test glucose levels and see if they’re stable. You may have to do it several times in a day to get a sense of peaks and variability, but you don’t have to do it very often once they’re stable. You get used to it and you can just give your kitty some extra treats after and you’ll be forgiven. My girl loves butter more than life itself, so I’ll often just rub my finger on a stick of butter and let her lick it off as a low-carb treat.