From what I see on the Spectogram you don't have problems of external noises. That's good. You also have low decay times which is also good. When I had no treatment in my room I was getting variable decay times of 400-600ms.
Your SPL is also pretty smooth between 35-75Hz which is a good achievement. Again mine was all over the place with no treatment, and with lots of treatment it was still quite a bit all over the place but much smoother overall. Only after adding EQ did I get flat and smooth up to 200Hz.
All room mode calculators seem to be for conventional rectangular rooms. This means if you have a room in the roof with angled ceilings it will give some miscalculations:
https://amcoustics.com/tools/amroc?l=420&w=386&h=240&r60=0.6
What to do? I agree with Tim that moving the speakers and listening chair may get different results. Whether they'll be better is hard to predict. Move then measure then repeat. If your room is symmetrical you could just move and measure one speaker first - arranging for REW to send the signal out of that speaker only. When I did this in my room I found that the best positions were around 'The Thirds' and 'The Fifths'. This is the positioning I currently use:
http://www.barrydiamentaudio.com/monitoring.htm
After that, add room treatment (have you already got some?). Then if you are willing, use DSP or EQ to complete the job.
One thing should be said and that is, do not get obsessed with getting a smooth response. Just do your best but the ultimate goal is of course to make music listening enjoyable. For example, some very narrow dips may not be noticed. What I've done so far is get a smooth response up to 200Hz using positioning, treatment, then careful EQ. I then listened to music and enjoyed it so much that I've not yet done anything above 200Hz!