You need a device to measure. Something like the U-MIK 1 a laptop with HDMI out and the free software REW would be the most beginner friendly setup. If you already have the computer, you only have to buy the measurement mic, around 100€, IIRC.
Then you need a device that is actually capable of 20-20KHz room correction DSP. Not sure if your current Onkyo can do that. No clue how good the "AccuEQ" is. However, once you can measure, you can measure the same signal with and without the EQ, to see what it does.
Just to reiterate: how exactly do you get the Dolby Atmos signal into the AVR?
You mentioned a blu-ray player that works. (Due to HDMI outs, these don't actually need HDMI 2.1 to make Atmos work)
Since your X90J is eARC capable, if you want to use ATMOS streams, the first order of business would indeed be the AVR. The 3700H has eARC capability, so that should work nicely.
My AVR support Dolby Atmos but the HDMI is only 2.0 hence there is no way to test the Atmos without the Blu-ray
Seems I really need to find the AVR with pre-out and HDMI 2.1 available first.
Thanks for your DSP measurement ability information. I would have a try.
But one question want to know, for the measurement mic, is it connected to the computer but not the AVR? Coz the AVR had the measuring mic for the AccuEQ measure.