I think you may have a complicated issue. It’s not just too dead, it’s got a mid-range suckout. I have heard this in rooms with too many Tube Traps. It happens when the absorbers perform too well in some bands but not as well in others.
Your problem may be that you aren’t taking out enough of the high end, so the room sounds too bright, and lacking in midrange only by comparison.
You may actually need to add better absorbers in the high end.
Diffusors are a good idea, generally speaking. placed behind and directly tot he sides of speakers they can really enhance imaging. One thing you can do is to try to create a quasi diffused pattern by removing some of the curtains you are using, at random intervals, with more removed towards the back than the front.
If you have any sort of tone control you can test this by reducing the treble to see if you find hte tonal balance more to your liking.
Measurements are good, but IMHO really hard to interpret for this type of issue.