Can anyone describe The Witcher (3) for me in terms of what type of game it is most like? For instance, how is the combat, and open world elements (is it a boring fetch quest filled world?)

It's a story driven game. It's pretty much "Game of Thrones" in another universe. It's based on a big book series that was "LOTR big" in Eastern Europe. The series' strengh are the story, the lore, the characters, but most especially their choice & consequence system. In TW2, about half the game completely changed depending on your decision and the developers claim it will be almost impossible to have two exactly similar playthrough in TW3.

Regarding combat, I'm not sure. Combat in TW1 & TW2 was mediocre (not bad, not good), but most journalists who played TW3 claim that it has vastly improved. Still, I wouldn't call these game "gameplay oriented" but maybe that changed for TW3.

As for boring fetch quest, one of the main marketing selling point was that TW3 is supposed to have very little to none of these and that most content will be story-driven. I am inclined to believe them because TW2 didn't have this type of boring content (which is why it's my favourite RPG)

And for someone currently finishing my first Dark Souls 1 run, so would I enjoy Dark Souls 2? The reason I'm asking is because I've heard mixed reviews, and I want to know what to spend my 60$ on

Depends on why you liked Dark Souls 1. The atmosphere and story are clearly not as good. However, if you enjoyed the gameplay and multiplayer, you should buy it as these are vastly improved in DS2.

I feel like DS2 is being shit on a bit too much. Basically, Dark Souls is a "game of the generation/decade" quality game and Dark Souls 2 is only a "Game of the year" quality game, and that's why people are being hard on it. But if you take a step back, it's still a worthy buy.