^I believe the most critical distance for time and phase coherence (an attribute that can only be found with classic Thiel's beginning with the CS series and a handful of other loudspeakers) is the 8' minimum recommended distance from loudspeakers to listener. The recommendation to start with an equilateral triangle has led many to perhaps over generalize that listener to speaker distance of 8' to a strict minimum distance between speakers. IMHO, still a good baseline from which to start. As rooms, and room nodes and reflections vary from room to room, other considerations might weight more heavily.
Have you considered long wall placement? IME, (and in a room not too different than yours) such placement, with absorptive room treatment directly behind the listener, has been the preferred placement.