I have turned into kinda an isic main over the last few months so I can offer some advice.
First and foremost, I do not think it’s best to play ISIC in every mode. The only maps I will even consider picking him is monuments (where he is god tier because of cheese) and Echelon (but Toby or Marquis would be a better ranged option here imo)
I don’t see the point of picking a sniper tank in capture or Faceoff. Those are pretty melee heavy modes and more like tdm and isic doesn’t have many options to deal with a rath dragon phoebe or Attikus rushing him.
As for leveling, you need to be focusing minions above everything. Your first priority should be the Shepard bot as soon as it comes into view. Your teammates will probably get to lvl 5 before you, but after that you should easily be able to outlevel and outdamage them.
For gear I recommend skill damage and regen. Skill damage affects your overcharge shots and regen lets your healer focus on the frontline more. Another good option would be shield strength or shield recharge delay.
Anyway, there’s my two cents.