I just got this game, but it seems like a build I'd planned to use for my first character might not be viable after all. So, a question: is it at all possible to run a character who wields two-handed swords and wears finesse-based armor? It seems like there's no real reason to wear it if you have enough strength to wear the heavier kinds.

It wouldn't be the best way because you'll need 14 Finesse for the best armor which is kind of a waste of 4 attribute points for less protective armor. Worse, Finesse armor will give you +Finesse bonus which won't help you. If you're building Strength, use Strength armor.

I don't want to build strength, is the thing. Since there are no swords that use finesse, I thought about a hybrid build. But if there's no reason to wear lighter armor if you can wear heavier instead, I might just ignore strength altogether and use a spear.

imo
if you want a trully resilient character, it should have a sheild, you might not see the difference in the early stages of the game, but later on, there are some pretty sick sheilds out there.

as for multibranch builds, the easiest way to have fun with those is if you do a 2 member party and use the skill that doubles attribute points you get by leveling up, that's by using thhis features that i had the most fun playing the game.

but back to armor, use armors that are on the same main attribute as your weapon, and if need be, use magic spells to add additional armor (or magic resistance).

hope it helps, have fun