And if Kratos can pick up a mountain, why does he walk slow when he's carrying a boar?

Because he’s following someone who walks slowly? Also I’m pretty sure his power scales with his rage. He can’t just casually walk around with a mountain on his back.

I've heard

Always interesting to see people write several paragraphs about the game they never played.

I mean sure, puzzles are simplistic but sometimes things you need to throw your axe at are hidden and sometimes you need good timing. The green wind things can be actually tricky. That aside it’s not even supposed to be a big part of the game like Uncharted (where puzzles are even worse) it’s just some minor challenge to open a chest.

The biggest example is Atreus becoming a sudden asshole and then suddenly becoming normal

I see you’ve never interacted with a child before. Jokes aside, there’s an explanation - he’s being possessed by his own powers or something like that because he doesn’t remember himself acting like that, which you’d obviously know about if you played the game. My only problem with that is that Kratos reaction to this behavior is milder than it should have been.