Since you are mostly focused on hybriding, i'd say don't get gunslinger armour. that set's pretty much only for people who will run full guns.

now onto the guns:
like bluescreenofdeath said, AP and sentenza lines are very situational. so i would recommend against getting it.
As a side arm, the polaris is one of the best (if not hte best) gun for a swordie (interrupts turrets with normal shots). but here are a few gun sidearm that you should also consider:

elemental: Magma or storm or nova driver.
each one of these have their own pros.
the nova driver is good everywhere. does about 1/5 more damage than the other two per shot (around 30damage, same damage as a polaris expanded shot). no negative side effects
magma driver has really good dps. as long as you can inflict a burn at least once, the magma will out damage the nova driver (the magma does 78-110ish burn damage per tick).
the storm driver's really good against tightly packed mobs as 1 shock tick will shock all the monsters touching the shocked monster. i was using a volt driver (4*) in FSC and i managed to do higher dps with it than using my polaris.

shadow: biohazard or umbra driver. for groups, it's much harder to use the biohazard due to its slow bullets. the umbra driver handles like a shadow nova driver.

you should aim at the left shoulder of the target (their left, your right side), just so the ricochet bullet will spawn inside the monster. similarly with charge shots, aiming slightly to either side can cause the ricochet to spawn inside the monster, doing ridiculous damage in the process. and one thing i've noticed is that the status alchemers inflict the status effect much much more frequently than polaris or biohazard. so you can actually count on the status being inflicted

should u decide to become a full gunslinger, i would recommend the blitz needle as your piercing gun. the callahan's easier to use but the blitz is better in most (if not all) situations

" i just tend to spam my polaris through the jelly palace or the last wave of an arena and to take down turrets, which serves me well enough, but im sure theres better ways of using it. "
looks like he's mainly using it for turrets in places outside of RJP. polaris is still a good choice if u keep playing this way. Also swordies tend to use their guns only for turrets (and maybe that last knight wave in arena). in both cases polaris is great

polaris is terrible for gunners (even as a sidearm). u are better off using a sword as a sidearm

I like it for a gunner. You don't have to get close and it makes sure nothing ever GETS close even if your back is to the wall. I use it rarely, almost always using Nova Driver instead, but when I need it I'm glad I have it. In FSC I have both on, and in very rare situations (beginning of the first room in Ashen Armory, for example) it outdamages Nova Driver.

Also, I didn't read the OP fully because it made my head spin from the wall of unpunctuated, uncapitalized text.

polaris as a main weapon:
i prefer the alchemers over polaris mainly because of the knockback of the polaris really interferes with your ability to hit them. low damage per hit (when compared with swords) in conjunction with slow bullets really kills ur dps. it also blinds ur teammates with the expanded shots. if i need to push things away, i use shield bumping and stun vials. so no need for a polaris.

polaris as a gunner sidearm:
doesn't really complement with a full loadout of guns.. guns cover a good range already, so the polaris's range isn't really an advantage. a gunner usually lack knockback power for whenever the mobs get too close. a sword is much better at knocking back things that are too close since the polaris takes a certain amount of distance until it can expand. also guns are great at doing sustained dps (well not really the polaris because of knockback) while swords are great at doing burst dps. thus a sword sidearm would complement a gunner loadout much better than the polaris.

@glowing-ember
for that first room in ashen armory, it feels like my team cleared it much faster when i was using an ASI med volt driver 4* than when i was using a polaris (was with the same group of people when i was using both guns). probably because of the higher frequency of shock infliction and lack of knockback?

ah. i was thinking that u are talking about the whole room lol. but yes u are absolutely right about the initial damage and the wall