Even if it did cause a minor spike in insulin, increasing adherence is of greater benefit. Ideally, plain water and electrolytes of course, but how many can honestly say they will stick with that? How many fasting-focused people here still drink coffee or diet soda?
Sure some sweeteners can cause insulin spikes in some individuals, plenty of YouTube videos demostrating glucose readings, but it's not universal.
In agreement with you, I agree it's cheaper, and better, to tailor your electrolyte-drink recipe and just make it in bulk. Make a small batch of the proper ratios, add in various sugar-free flavorings, see what flavors mask the taste the best, and then make it in bulk and store in a travel container.