So I'm quite a ways into The Witcher (maybe about 38 hours - chapter III somewhere), and I realized there are two CD Projekt Red-developed modules included in the Enhanced Edition: The Price of Neutrality and Side Effects.

Are these canonical to the overall story of the games? Or are they unrelated one-offs? I'm curious if I should play them.

They're unrelated. I played them after I'd finished the game. They both take place before the events of the main game but aren't really related in any way. The Price of Neutrality is essentially a re-telling of one of the original short stories, although it changed a lot of things. Side Effects is it's own thing.

They can be interesting if you want some more Witcher, but I'd recommend continuing on with the main game for now!