That's a generous sized room. The main thing to remember is you are treating a system that consists of the speakers, where they are in the room, and thus how the acoustically load the room, and the room characteristics themselves, including all walls ceiling and floor. No one wall or surface is more important than any other except for two things: 1) Controlling first reflections on the side walls and ceiling. 2) Bass traps are different, as corner placement is far more effective.
Acoustics isn't opinion, it's applied math and physics, and it's pretty well documented and computer modeled at this point. A room of your size will require somewhere between 100 and 140 ft2 of 2" acoustical panels between the 4 walls and ceiling, plus 2-4 Bass traps. Figure $12-15 per ft2, installed for 2'X4' panel or around so maybe $2500 all in, depending on your choices of fabric and local labor rates. The panels aren't hard to DIY install, but it's definitely better with 2 people and appropriate ladders. A pro will also use a laser to make sure panels are installed square and level.
Take your time, do the math and enjoy the results. You'll wonder why you waited so long.