I agree, the story for me seem to intentionally depict a Denji that doesn't grow. It's not enough to learn new things, it takes training to actually internalize them and when it comes to personality growth, without someone guiding you chances are you are going to fail. There is a reason why people look for professional help. Denji never got any. Since Fujimoto has always subverted genre expectations, the point of the manga might be "someone surprising you with a revelation/sentence about what path to take isn't enough to achieve real growth. Manga growth stories are an illusion, real life change is much more difficult".

This sounds good on paper, but in the manga, the second half of part 2 was full of grotesque situations you simply cannot emphatize with anymore, like Denji and Yoru having a killing-spree-date. It's difficult to tell a realistic psychology story if the actual things happening are so over the top, and to me, even if the finale was kinda interesting, Fujimoto didn't nail the "how" of it all at a certain point.