I'm going to have to echo Frostjaeger and put my vote up for Engineer... or at least however you've renamed the archetype of the Technical Sentry.
I think, for the average SARPer starting out, that it's one of the best occupations since it gives you something to do in the majority of situations involving a plotship.
Ship combat? No doubt you're busy helping the ship stay together, or contributing to give it this extra edge?
Boarders? You're one of the first line of defense against both the dangers and the havok they may cause.
Away missions? You're pretty handy with sidearms too, and can leap inside a power armor as needed. Extended stay away from the ship? You have the knowhow to fixup that shuttle, power armor or accessory.
Equipment touches every crewmember of the ship, so, you have plenty of excuses to interact with people, even the officers on the bridge if an item requires maintenance. Like IT, every branch relies on you to run well. Which, indirectly, makes you learn on the job about every branch too.
And, once you're truly settled and acclimated to SARP/in the plot and that you're participation are allowing you those rank promotions, you get to fairly easily orient yourself toward a more senior ship position. The easiest is tactical (since you're already kind of a combatant), but you're an handy person to fill in any vacancy a GM feels he might have higher-up where he thinks you might shine.