While it sounds counterintuitive, side wall reflections impact the width and depth of your sound stage more than I ever imagined.
As you consider room treatments, read a few articles written by Art Noxon, owner of ASC (Acoustical Sciences Corporation) Tube Traps.
I like his tube traps because you can literally “dial in” the soundstage by simply adjusting the amount of diffusion and absorption you need.
Use a mirror to find your 1st and 2nd side reflections, and watch your soundstage adjust to meet your needs.
Hope this helps.