They make 18.5" 590A1s. All the 590s are 20". The A1s have different features. Metal trigger group and metal safety.
Either way you have the same issue. The mag tube will not allow a 18.5" barrel. You have to replace the barrel and the mag tube. To do it you may save $50-$100 on the cost of buying a new 500 the way you want it.
The barrel is easy to change once you have the tube right, Might call Mossberg and ask them how you change it. If you want rifle sights and speedfeed as you show the price will go higher.