Reducing sweetness of packaged cookie mix

Probably there is no such way, at least not one that's worthwhile.

First, there is the problem that designing a recipe well is a skill which very few people have. Experienced bakers can progress to it, but inexperienced ones can make 100 trials but won't understand what went wrong with any of them. Unless you're in it for the fun of it, it's easier to find another recipe which works for you (you might have to bake through 4-5 failures to find a good one) than to redesign an existing one (might have to bake through 25-30 failures to make a good one).

Second, you're not starting from a recipe, but from a mix. With standard ingredients, you know what went in, and can change the ratio easily. With a mix, you have no idea how it works and how changing something will tip the balance. A simple addition of one more ingredient (e.g. flour) will certainly not cut it. If you add enough to make a change in sweetness, you won't like the change in texture, which, for flour, will be rock-hard cookies.

And this brings us to the third problem. Sugar is not a sweetener in cookies. It is a bulk ingredient which gives them a cookielike texture, and the sweetness is a side effect. If you combine the same ingredients but with less sugar, you'll end up with tiny cakes instead of cookies.

Proposed course of action: forget these cookies. You don't like the taste, and you can't keep the texture while changing the taste. Baking cookies from scratch is not much harder than baking from mix. Go to a good recipe site, find a well-reviewed recipe for the type of cookies you prefer, and bake away!

Balancing the sweetness with something that adds textural or flavor depth can mitigate the cloying, one-note sweet sensation.

Try adding ground nuts - not too much, otherwise you will mess with the recipe chemistry.

Try sour (or perceived sour) flavorings like citrus zest or mahlep. Amchur or Anardana could work too, given they are solid sour flavorings, have not tested. Sodium acetate, the vinegar flavour used for potato crisps, would probably too... vinegary. Careful with sour liquids, there is probably baking soda in the mix that you might prematurely activate that way ;)

Try the classic baking spices (cardamom, nutmeg/mace, allspice, anise; cloves and cinnamon could work but could also make it appear even more sweet).

Try hot spices - ginger or even (little!) chili pepper or black pepper.

Gingerbread, for example, can be rather sweet and uses both baking (all of the above ;) and hot spices - ginger and not infrequently black/white pepper.

Try adding unsweetened chocolate.

"Thinking out loud" here. Wonder what a tablespoon or two of almond flour might do? I always sprinkle the top of brownie batter very lightly with coarse ground salt. The whisper of salt with the chocolate is really delicious. I wouldn't be afraid to add 1/8 tsp cayenne pepper to the batter while mixing either.

I have a Ghiardelli brownie mix I found too sweet. I removed 1/4c of the mix and replaced it with 1/4c cocoa powder. I also replaced the called for 1/3c of water with the same amt of coffee. Turned out very good for me!