About to invest in room treatments; GIK, RealTraps, DIY -- what is your experience?

Check out Acoustic Sciences Corp ..  https://www.acousticsciences.com/   Their stuff has been an integral part of my system since 1993.  https://www.martindareff.com/Other/HDR-Treated-Images/i-ZmLg23s/A

With regard to diffusion vs absorption with GIK panels, it costs only a little more to hedge your bets. I was initially skeptical about diffusion, since I was trying to tame a very bright room. I ordered Alpha panels with the optional cloth finished back so that they can be used for diffusion + some absorption or pure absorption, depending on which side faces into the room. I found that, in my room, the diffusion was noticeably better.

I have a total of nine GIK panels (3 Alphas, 3 non-membrane bass traps, 3 membrane bass traps) and haven’t encountered any QA issues, other than the little adhesive backed GIK labels eventually falling off the cloth covered panels.

I initially sent room measurements taken with a calibrated USB microphone and REW to GIK for recommendations to start off the process.

As expected, the improved reverberation time makes the room act as if it is much larger. Speakers (Sony ES SS-M7) generally considered to need lots of space to work well never sounded better...

I have a similar ceiling height.  I got Vicoustic bass traps and ceiling panels at first.  I was ready to get some wall panels, but Vicoustic kept changing their design for hanging/adhering to the wall.  After 4 months of waiting my dealer gave me a refund and so I bought directly from GIK.  I agree with martin0820 - the diffusion is just a little upcharge over absorption.  If I could do it all over again I would have gone with all GIK.

I had a great experience with GIK. I chose them as well-established science based and reasonably priced but was tempted by synergistic research stuff. GIK = good tools, good advice, and great results. I have a vaulted ceiling with speakers along long wall and top of vault along center of room in long direction. Ended up with 2 244s on ceiling, soffit traps stacked in front corners, etc. 14 piece total but only used 12. Dealer said he couldn't hear my room at all which was great. Not dead sounding at all.

I went through a very similar situation as you presently have.  Over the course of a couple of years, I read the books and literature, plus participated on Gearslutz acoustics threads.  That was most helpful.  It's good that you've done some REW sweeps; it was valuable to be on Gearslutz in order to properly interpret the various sweeps and to know what they indicated in terms of treatment application.  

Perhaps I misread your approach, but it did not look like you started treatment with very low/low frequency treatment (which you should).  My room is about the same dimensions as your room, except for 9' ceilings.  I ended up treating the corners, floor to ceiling, with RealTraps Mega units, as GIK were not as effective as I needed.  All along  the rear wall (short wall), I use 10" Knauf  mounted in skeletal frames behind RealTraps diffuser panels which are sitting atop their  2' x 2' x 6" absorption panels.  This will give me pretty much what I need for LF control and diffusion.

I moved my speakers closer to the front/short wall to help speaker boundary/LF issues.   I'm testing the first reflection/side wall areas with near diffusers and with combination of diffusion and absorption/range limiting to preserve source content.  On the front/short wall, in addition to the 34" wide corner Mega traps, some additional very effective absorption is necessary; I think you'll find the same.  

Ceiling area just above and toward the front of listening position is something you can determine with additional sweeps and experimentation.

The reason I used RealTraps corner units in addition to their effectiveness and quality build, is that I just did not want to build those myself; predictive software models weren't what I would want to use to build, etc.  However, for non-corners, regular rectangular absorption panels are a piece of cake to build, and pretty cheap.  Knauf from Home Depot comes in pieces that fit just right, at less than 20 bucks a panel, and in thicknesses of like 6" (minimum you should even consider) to over 12".  Knauf has tested very, very well.  A simple skeletal frame is cheap and easy.  You'll save yourself around $1,000 or more for the amount of absorption you will need in your room.  In order to afford the RealTraps units, I had to save money elsewhere.  That said, a few GIK 6" with range limiters was also something I figured in my scheme.  

I suggest you consider calling Ethan Winer at RealTraps for advice.  You can take it or leave it at the end of the day.  He is very nice and quite patient.

Take your time.  Address one issue at a time.  Keep running/interpreting room sweeps/measurements, then move to the next issue as indicated.  Be careful of over absorption or using where not called for.  Move your listening position and speaker distance from front wall as necessary.