I don't believe the "Cheat Guide to the World", while it is OP, is a bad thing for the story. We had quite some stories, in which the MCs somehow gained OP cheat abilities, that are worse, yet the stories range from really bad to awesome. This one is dangerous cheat item, as it may fall into the wrong hands! (Remember, he was the only one to discover how to read the old sacred language. This indicates, this book was meant for him from the start. Sadly he taught the way to others. Mainly that priestess bitch.) Other MCs have a experience gain bonus of 400-times or an affinity for all elements of magic, which they suddenly may combine, or do even poses a skill that breaks every balance in their world and let them gain everything. This guy now has access to all tools and the knowledge on how to defeat the demon lord (it even reads so in the title of the item). That's not too broken. That's fine. And it leaves heaps of non-shitty asspull-free ways to add drama. A good setup IMHO.
Also, eat it tsundere. I also like tsunderes, but not the kind, that are mentally abusive and even betray their love interests (harm them for good), as she did by talking shit about him and critisising him. She wants him to stand up for himself and to shine
? Then she shouldn't help to keep him down, when he tries to talk back! What did she expect? Talking back isn't good enough? Should he have attacked that douche and later be labeled as traitor? Wanting the MC to be a better man, but denying him any plausible way to do so. "But there are always ways!" Yepp. Some she made up, but that don't actually make sense or are impossible to follow through. I believe her to be that kind of unreasonable character, that literally wants her Daddy to pick a star from the sky. Only a few ways to handle this. Either fool her or distract her and make her forget about it. That is, if you care. If you don't, just set her aside and leave her in the dust. Good call MC.