First, the drapes combo with a window is fine, I mistakenly thought the drapes were intended for for wall covering. No reason to think that either, my mistake Bill*.
Above a drop ceiling, is ideally suited as a highly effective bass trap when filled with fluffy batts, or equivalent insulation. For starters, I've seen just the effect of a one inch tile covering a 16" open space, ... and it's a very nice and effective bass trap that absorbs pretty deep due to the physical distance of the fiber board away from the boundary. Throwing some inexpensive insulation up there makes it nearly ideal.
It's ideal for two reasons, it works so well so cheaply, and it occupies no space in the room. I've seen several before and after measurements that clearly illustrate the benefits in both time (ringing) and freq (smoothing) domain.
Oftentimes, the grazing angles off the ceiling preclude it from addressing reflected energy in the MF/HF, so perhaps a small ceiling hung cloud would both maximize the room's bass damping, and absorb the critical reflected bands within the first dozen or so millisecs. You've go the makings of a nice ITDG, which means a nice separation between the direct and reflected energy. Initial Time Delay Gap, is the gap right after the direct sound, and until the first reflected energy encounters the LP. With the sidewalls spaced nicely apart, it leaves the area directly behind your head, ceiling between you and the mains, and of course the floor reflections.
In your case, with the sidewalls so distant, the debate on absorbin or diffusion for lateral sidewall energy isn't as significant as in typical orientations that are longer than they are wide. That said, the remaining early reflected (from above, below, and behind) energy can be soaked up with absorption with no ill effects on spaciousness. Sidewall reflections are fine as long as they are reduced in level, or delayed in time. Our ears lateral spacing dictates there's little good that comes from the ceiling and floor early reflections. So fully attenuating them is no problem and often clears up other details via lowering the smeared in time early reflections.
Bill's right, big windows, sliding glass doors are lossy diaphrammatically in the LF. I've got one to my right, it's quite effective really at bass transmitting out. That wasn't my initial concern, and drapes are fine there. It's the area directly behind your listening position, and it's critical. Total absorption at that spot (if it's 4" that's fine), then as much bass trapping as aesthetically possible, then measurements, ray tracing, ETC determinations, those results would dictate further moves.
Subwoofer optimization aside, a properly damped bass trapped room will likely be a revelation. Rooms can easily possess 20-30dB swings in response, modal issues, ringing extending way too long,...cleaning that up will help clarity, and tonality. The great thing about bass trapping, the cheap, loose fluffy variety is found to be more effective with thick traps than the rigid stuff previously believed to be best. If you consider fiberglass, rock wool, polyester, or cotton, your options are plenty.
As far as effectiveness; If you're using 4" of space, you use a rigid product, so you're cool there. But if you've the space for very thick traps like elsewhere in the room, utilizing fluffy insulation config'd in a manner so as to not compress much, they're most effective and least expensive. You can certainly use rigid 703 style, or the eco denim cotton stuff too, ... in a stacked superchunk method, which countless examples are out there. It too is effective.
Be mindful that porous insulation, like the items I've mentioned, is a velocity-based absorber. So it needs to benefits greatly from being spaced away from a boundary. This puts it into position for it to be most effective. The problem is,as you go thicker and thicker (for lower and lower freq damping) you need to transition to a material with a lower flow resistance, like the pink fluffy etc. Next, the issue becomes assuring the insulation doesn't become too compressed via implementing various techniques, supports etc. Methods include stacking chunks in the corner with bird netting supports lessening compression of lower layers. Another is hanging vertical sections fluffy from above, allowing them to hang freely and never compress.
Despite it's measured superiority, in many cases individuals prefer using the more rigid super chunk approach which is as I stated above, still an effective approach.
Loose rule of thumb to maximize effectiveness;
If you have 4" to use, 4" of 703, having 8" of space, use 2" 703, 6" fluffy (or 8" of Roxul SafeN-Sound), 9"-12" or more, utilize fluffy insulation.
Another item, what I've found (that's consistent with much other work/whitepapers etc), the front wall can be entirely absorptive with little to no negative side effects. This is lower on the order of importance, but I certainly like it.
Recap; the first two are a win-win, no downside scenarios
1.) I'd put a 4" (or as thick as possible) directly behind the LP
2.) Install loose fluffy above the drop ceiling
3.) Measurements, become adept at them and everything should be based on them
4.) Find and treat detrimental first/early reflections
5.) Find and treat modal and boundary related problems
My 2 cents, my opinions/experiments and findings. Much coincides with that of the smart guys/acousticians. *I hope my mistakes haven't found their way into this post too! I'm undergoing some serious, life changing health issues, and unfortunately I've taken a hit intellectually,..a pharmacological cesspool if you will.
Best of luck
Some interesting reads;HereHere