I've decided I want my character to use two katanas and chose my character's statistics, class, and proficiency towards that. I picked fighter, not kensai, since it looked like that would work best. Should I have picked a different class or proficiency for the greatest potential power?

Go for the Berserker, with "enrage" ability your fighter will
resist all mind spells + stun, etc and gain 15 hitpoints temporary,
but there are also disavantages, so read the kit description.
For the weapon proficiency I will choose "two weapon style" ++ and
longsword++++. But it best you follow your own taste, good luck.

You can get the celestial fury +3 - in my opinion, the second best weapon in the game - very close to the beginning. After you see Gaelan Bayle, at any time you feel comfortable doing so, you can pay a visit to the guarded compound in the temple district, fight a lot of people, and take the katana off one of their corpses. Katanas are fine. Congratulations for picking a fighter over a kensai, too - that means your fighter will double as a tank as well as a fast, efficient killer. The deal is, kensais kill faster than fighters, but get killed much faster than fighters, as well. Just keep going, you'll see your decision was a wise one soon enough.