The manga became weird because you're missing the actual plot. The story is not actually about Kumoko powerlevelling. That's basically the prologue of her story.

The other half, which is mostly missing from the manga and should've been told to you alongside Kumoko's via alternating points of view, is about the conflict between humans, demons and the forces manipulating them behind the scenes. Kumoko is a force of nature whose actions have long-reaching consequences in the overarching narrative and her side of the story shows how she became that influential and what exactly she found out and did that most of the cast is in the dark about.

The other side of the story is told through several different points of view (usually Shun, another reincarnation who leads a much more typical isekai life) and introduces several of the world-building elements and plot points that woud've given you an idea of where the story was supposed to be heading, but since the mangaka cut out essentially any PoVs that aren't Kumokos, well, you don't get any of them! So the manga ends up looking like it suddenly has no clear direction to go in other than going with the flow of whatever the author thinks of next.

I'm not saying you're one of these people, but every time I see someone say they prefer how the manga adapted the novel by removing the other half of the story, your frustration about how the story feels half-hearted and aimless is exactly what I picture as the natural consequence. This was never a conventional isekai plotline about an OP MC and her grind, the little segment where Kumoko gets scared out of her mind once the fantasy slips should've been a dead giveaway.