Indeed, if the amounts in the two envelopes are x and 2x then your expected payoff is 1.5x, but here x is not the envelope you chose, it is the amount in the smallest envelope. So I agree with everything here, except the last sentence about the advantage of switching - noting that the expected payoff is 1.5x doesn't tell us anything about the advantage of switching.
Indeed, if the amounts in the two envelopes are x and 2x then your expected payoff is 1.5x, but here x is not the envelope you chose, it is the amount in the smallest envelope. So I agree with everything here, except the last sentence about the advantage of switching - noting that the expected payoff is 1.5x doesn't tell us anything about the advantage of switching.