You wrote two things that stick out to me:
"What they describe and what they do in action is very different."
"You're right, it may be their version of poly may just be different. Maybe we're asking for too much too soon. Maybe we're asking for too much in general. Maybe we're over thinking this. I don't want it to turn into a sexual relationship only and the drop off in communication is concerning for me."
It sounds like it is clear that they want something different than you. So if we were to take that at face value and nothing more, then you have two simple options. You can let go of your wants/needs/desires and enjoy what they are comfortable giving in the way that they want to give it or you can state clearly what you want and need and then let the "chips fall where they may."
I run the risk of over-simplifying all of this, and I do not intend to demean its subtleties and complexities. However, you can control your actions and not theirs.
Spending time together without sex is a fast-track way to determine if the spark of chemistry that you feel is "sustainable" without regular hot sex. I have known many people who had fantastic chemistry on the dance floor, but when they took it to the real world it fizzled quickly. As such, you could have fantastic chemistry when regular sex is involved but not when real world concerns are addressed.
For instance, will he still be interested if you have to talk regularly about "mundane" things like work or family or chores or _____?
It may be that just meeting him where he is and letting go of your bigger longer term dreams or ideal version of your relationship for now is what would let him settle into a relationship with you.
Or it could turn out that you feel like you are prostituting yourself--selling part of your soul that you really need to be recognized and responded to.
Only you can know that, of course.
Keep us in the loop. I will be interested to hear your process and what happens.