I'd like to make these muffins a little less sweet. If I reduce the sugar, should I make any other adjustments? Thanks, everyone. ;o)

I think of sugar as a quasi liquid in the baking context. I would add the amount of sugar you prefer, then see if the batter needs adjustment in either the liquid or solid direction.
I do often reduce sugar, especially in muffin recipes. Usually no adjustment seems necessary, but it depends on all factors. Your experienced judgement should be the best guide.

Since you don't have an answer yet I'll tell you what I'd do. Reduce sugar to 3/4 cup and use a low protein flour. At least as low as bleached AP. I think that should be satisfactory, but report back. These sound very good.