> Can we agree that we both exist, but we have differing models for what the necessary constituents of that existence are?

Of course, yes.

> I can certainly imagine creatures with brain-driven consciousness but without a soul

This would probably be the heart of the disagreement. I don't believe this is possible. Such a creature would not have qualia.

And, as a species, I don't think we're any closer to resolving this question "objectively" than we ever were. fMRIs say where blood is flowing in the brain, but that's hardly enough to explain the phenomenon of subjective experience.

By the nature of the question, we won't be able to attack it from the outside, and I don't think I could generally convince another person that they have a soul that exists, if they're inclined to explain themselves using materialism, which at this point has become flexible enough to be unfalsifiable, with the everlasting faith that someday science will fill in all the gaps.

That's why my approach now is just to poke holes in the seemingly impenetrable confidence that materialism is the only "rational" way to think.

(By the way, I'm not saying you hold that position.)