Don’t pull a chapter out of the middle of your story. Agents will be suspicious. They’ll assume that the beginning of the book isn’t interesting enough for you to show off. In their minds, your tactic is as good as admitting that yourself. If you can’t hook your readers with the first chapters of the book, they will never reach the middle. The same with agents. They want opening chapters that hook them so tightly they rush to ask for the full manuscript. Your concern that your opening chapters aren’t as strong as your middle ones is your red flag to go back and make those opening chapters great.