You seem to be arguing that women who do sex work are under "economic duress" because the alternatives are so awful, therefore we should make it illegal, forcing them into back those awful alternatives. That seems backwards to me. Surely we should be banning the shitty, coercive minimum wage jobs, not the sex work women are choosing in preference to them?
It sounds to me like your distinction is that if any person makes less money than any other person, then the first person is under economic duress and thus is incapable of making any voluntary choices.
Sex is not like every other human interaction. If you forced someone to answer phones for eight hours, it is unlikely they would be traumatized in the same way as if you raped them for eight hours. Its different.
Not every transaction between people in unequal economic circumstances is suspect, but we're not talking about every transaction. We're talking about something much more intimate and emotional. Such transactions are far more suspicious.
You're moving goalposts.
Obviously there's no clean line as to how the alternative courses of action have to be before it counts as duress. But we don't have to draw the line clearly in order to be able to make the distinction.