I had a similar room and wanted to make my Thiel CS3.7 work in this room. I managed to do so with GIK and then DSP using ROON Convolution filters. Adding more GIK panels and moving reflective surfaces as far from the speakers as possible allowed me to eventually remove the Convolution filter.
The sound was about 80% as good as the Thiel CS3.7 could do in a more spacious room. The speaker could not really breath but the sound as not fatiguing nor unpleasant, just not the best it could do.
I recently put in the KEF LS50 Meta with the KC62 sub and kept the GIK panels. The sound is fantastic, so much better than the bigger CS3.7. The LS50 does not sound like it is being held back.
So I think putting a restricted low frequency speaker in a small room is the way to go. You can augment that with a great fast sub like the KC62 if you want more low end.