Room Treatment Advice, Interpreting Measurement

Took my first measurement with REW... still learning what it all means but I see some remarkable drops in spl at a somewhat repeating pattern.
Curious to hear what some of the knowledgeable posters here think might be going on and what kind of room treatments I should investigate.
Room is roughly 20 by 40 with cathedral ceiling.
I completely understand that recommendations based on so little information are difficult. The problem is that I know so little, I don't know the right questions to even ask yet.

SPL.jpg
 
at a somewhat repeating pattern.
Did you average multiple sweeps?
If there are timing differences that is what would result.
Search "multiple sweeps" here:
https://www.roomeqwizard.com/help/help_en-GB/html/makingmeasurements.html

If you take one nearfield and one farfield measurement the difference should mainly be the room.
For confirmation you can take several measurements at different distances, the room influence should (very simplified) correspond to the distance to the speaker.
 
My guess would also be: here's 2 speakers playing the sweep, microphone a little off centre. With one speaker playing at a time that repeating dip pattern should be gone. You could also move the microphone till its in the exact centre, but it requires a bit of patience 🙂. Better to look at the impulse (IR) for that.
 
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