You don't know how you're going to place until you show up and do it... unless you wait for Masters Nationals which is almost a year away, you're going to be in the same boat (so to speak) regardless of which national show you decide to do.
The advice that was given to me was that the more shows you do, the more the judges get to see you so if you want to continue dieting and hit Nationals, go for it. The best thing that can happen is that you win your pro card. The worst thing that can happen is that you don't win, BUT you can obtain feedback from the judges to find out what they're looking for and come back next year and blow them away! In my opinion, that's a win-win!
Good luck in whatever you decide to do!
PS: For the record, I think that physiques tend to win shows and age doesn't have anything to do with it. There are plenty of "masters" who win their pro cards at shows other than Masters so I wouldn't view that as a negative for you whatsoever.