I want to be a marine interdiction agent with cbp, should i wait for an opening or just get any CBP position?

I want to be a Marine Interdiction Agent with CBP. I will have my OUPV 6 pack Captains license by 20 July which is really the hardest condition to meet for the CBP MIA position. I do not know how often vacancies open up for the position but I remember last year there were quite a few. My question is, would it be better for me to secure a position within CBP and Transfer when there is an opening for MIA, or is that even possible? Or would it be better to wait it out until there is a MIA vacancy open and apply at that time. My situation is I am 29, CBP cutoff is 33. So I do not want to run the risk of being less desirable.