Room Treatments to Minimize Interactions

Could use some recommendations on where to start with room treatments to optimize my new speakers, they are the VA Mahler. My room is 33’ wide by 21’ deep. The speakers are currently on the long wall about due to room usage issues. The speakers are setup about 18’ apart and 19’ from the listening position. With this layout I am getting very good sound-staging width and depth. The speakers are 3’ out from the back wall, which is sheet rock (I can’t move them farther out due to traffic flow through the room). My listening position is a large couch, with overstuffed pillows, up against the wall opposite the speakers, so my head is about three feet out from that wall, which is a series of glass windows that open partially into another room. The ceiling is 10’ and flat. To be precise, due to the number and size of openings in this room on most of the walls, the 33’ by 21’ size is rather understated. There is a wall to wall built in bookcase to the right side of the speakers, about 6’ away and a free standing masonry fire place that opens into another room on the left, about 5’ away. The only furniture is a couch, loveseat and coffee table opposite the speakers on the listening position side of the room. Both the wall behind the speakers and the wall behind the listening position are fairly reflective. This is likely way more information than most of you want to hear, but I thought it might help someone give me a starting point.