Since you are doing fine with your plate shield, how about a shadow weapon?

A bristling buckler works perfectly fine in areas with mostly pierce damage, and you can always switch out to your plate when you go to other areas.

Supernova: normal, expanded bullets, always knocksback on expanded bullets. Best used for survivability
Blaster: Normal damage, 3 shots, a decent speed. The 5* upgrade has a small knockback on each hit.
Polaris: elemental, expanded, chance of shock. Best results on turrets or enemies prevented from knockback.

Faust: Heavy sword, 2 swings, worst charge, only weapon that causes curse. Does more dps when hitting groups of 4+

Nightblade: Med speed, 3swing, great charge. Does more dps in groups of 3 or less.

Personally, I use the faust because heavy swords are my style. But a nightblade is safer in most occasions with the faster swing and faster charge.

The gun question depends on what you prefer. Do you need a kiting weapon, to deal dps while keeping you safe? Or do you want a turret killer, with superb knockback to literally keep everything away from you?

The blaster is normal damage. And being normal damage, it isn't as effective when used by knights with more than 1 weapon slot.

The polaris line. It deals elemental and chance of shock

Shield-wise I would recommend having at least one shield for each damage type: Elemental, Piercing, & Shadow. This is especially important when you only have one armor set (and cheaper/faster to obtain then 3 armor sets). VPS is fine for FSC, but it is not very useful elsewhere in T3 (OK, Unknown Passage, but that is a rare event). I would recommend a Grey Owlite Shield over the VPS, because it is more useful for T3 in general and it is useful for more boss runs (FSC, ICMF, OCH). For a Piercing Shield BTS is the obvious choice for swordsmen otherwise the Royal Jelly Shield offers more protection. Shadow leaves two choices, Dread Skelly Shield and CoA. Personally I went Dread, just because it was quicker to get (and a very solid shield) but ended up also crafting a CoA for T3 Fiend Shock/Fire arenas.

Handgun. One other thought would be the AP. Ever since it went pure Elemental it is also a great turret buster. The Polaris might be better vs turrets, but an advantage to the AP is that you can spam it in parties without worrying about annoying teammates. The Polaris needs to be used with care when in parties, so as not to send monsters flying into teammates and obscuring vision. AP has less damage per shot, but has high DPS due to less reloading: Shoot, Shoot, Shoot, Shoot, Shield Cancel, Shoot, Shoot, Shoot, Shoot.