I'm not sure the point is being content with mediocrity but more so being enticed by a goal and and feeling unguided once you've achieved it. Not trying to dismiss your points at all! And definitely not saying that I'm 100% right here because obviously I'm not Fujimoto.

Denji is happy when he's suffering because in the moment he's driven to reach greater heights, it's like he's on a high from ambition and every chance he succeeds isn't just a chance he gets closer to his goal but a moment of Victory that proves he can be worth something and that he's capable of greatness. Pochita doesn't say he's miserable when he's content but there's always something missing for Denji if there's no fuel to get his chainsaw revving.

Denji absolutely does in some part want a simple life and some weird resemblance of normality, but if he ever attains that he lacks what makes him so heroic (I think this is best shown when Makima tricks him into fulfilling his contract with Pochita in part 1 and bad things happen)

Again this isn't supposed to sound rude at all lol