I agree that I really don't like Andy Weir's character writing: his dialogue in particular is rough, but despite normally characters being what I'm there for in stories, I give Project Hail Mary a bit more credit than this. The story has some interesting ideas and I think Weir's strength is the mystery of process: you see a challenge, and then you get to enjoy the competence porn of someone successfully going through the process of finding a solution, and I think he nails doing that in a really engaging way.

I do think the movie will probably end up better than the book: having a screenwriter go over the dialogue alone will do a lot, I think.