What process did you use to integrate multiple subwoofers for 2 channel listening?

I think you've nailed it your thread description - trial and error. It takes a lot of time to go through the various iterations, but viewing the measurements is well worth the effort.
If you haven't done it already, go to AVS forums and to the Rythmik thread and download Enrico's tip for dialing in their subs (it's in his signature).
Don't be afraid to use different volume adjustments, different frequencies, different bass extension filters for each sub. Experiment with room locations (ie different angles, even pointing the front of the sub to the wall).
Take notes (I added them to REW). It's all important so you know what works and what doesn't. Taking notes will help speed things up. Carpet slides are a great idea.

@hilde45, Can you just do 10-200Hz for mains and mains + subs please.
At full range, the level of detail gets lost.

For starters, I would only use the 2 Rythmik subs to begin with. I think once you have these dialed in, you can add the REL sub to see what affect it has, and start tweaking from there.

If you download Enrico’s dialing-in document, you’ll find you need to set the Phase correctly, and Volume match your subs to your mains (pink noise and SPL meter required). Without doing these, you’re wasting your time with guessing. If you do this, you might find it easier to fix that early dip (use the extended bass filter to see how you can bump it up). Don’t adjust both subs the same all the time, experiment with one sub only, then set it back and try the other sub.

You should also load a graph of your main speakers only. I think you’ll find the same results as the graph you posted for anything starting over 150-200Hz (I’m not sure what the cut-off is on your subs). Basically, the subs aren’t going to fix this.

This is without any DSP, just location and adjustments on Subwoofer - https://imgur.com/a/G9kHdTt

I had to back off the low end frequency (20Hz - 30Hz) because my wife was complaining it was making the house shake too much.

@hilde45

Can you replace your old graph with a new one, and include measurements for the mains only. I think you'll find those nulls (127Hz, 208Hz, 244Hz) will remain unchanged with or without subs. This is where DSP might be of help to you if you're looking for perfection.

@hilde45
I'm curious, can you post what your settings are on the 2 Rythmik subs please, thanks.

Thank You.