Honestly if you are planning on purchasing 1 of every expansion and are interested in competitive play enough to want to purchase extra cards to run certain lists then I think a better starting point is which ships you want multiple copies of. Multiple copies of an individual ship could be far more impactful to list building (or just plain fun) than extra cards.
Chances are that by the time you have purchased multiple copies of ships that you want anyway that those card gaps will be filled in much more anyway.