To a natural human way of thinking, our existence suggests some kind of 'soul' on first reflection
I'm a natural human (or naturally human) and such a concept never occurred to me. My parents never spoke of such a thing. Only when other kids brought it up, did I hear of it, and even then, as now, it seemed to me just complete nonsense.
we can't know that 'soul' is a myth
That makes no sense.
the problem is that you are taking something there is no cause to assume
in the first place,
and then saying that we can't know it isn't true.
Suppose a fellow has a definition of "soul" and there happens to be, yet undiscovered, some phenomena in the universe that perfectly matches that definition. Even if that were the case, that fellow is not talking about said phenomena, only talking about his imagined definition.
So, you can draw up a definition of anything you might imagine, anything at all, and that's fine
but you can't logically then turn around and say "we can't know that it's just a myth"
unless you have something other than a lot of cultural baggage to suggest otherwise.
I mean, where would you stop?
Buttered singing elephants.
If I say there are buttered singing elephants,
even if there is no evidence of buttered singing elephants,
nothing that actually suggests that buttered singing elephants exist,
would you say. "but we can't really know if they exist or not?"
Well, you could say
that, but what meaning would it have?
If you argue that so many cultures for so many centuries assert a soul.
why would that have any bearing on it?
Why would a million people believing something make it any more true than one person believing it?
truth does not depend on popular vote.