I'm sticking with my Blade for now in chapter 3. He's pretty good at the level I am at. But he has to be the one using the girdle of hill giant strength.
My question is what party composition is best. Right now I have Korgan, Minsc, Jaheria, Viconia and Jan Jansen. The paladin might be good to sub in for Minsc to use the Holy Avenger, but then he would have pretty low strength. I also want to get someone who can use katanas, but the ranger who is proficient in them also has low strength. Do I have the optimal party given my starting character?

I would keep minsc. He levles up very slow but is also a great fighter. You could get Yoshimo in the copper cornet in the slums if you want someone who can use katanas. It kinda depends if you like to play the game using more strong warriors or mages and clerics. If you do the quest for Renal Bloodscalp then you can get Edwin. He is a lvl. 9 Conjurer but with a ton more spells than a normal lvl 9 mage. personally. I dont like paladins, so I would keep minsc, but If you like them then you can get anomen (cleric/fighter) but he seems like a paladin.