God’s Design CAN be used like you’ve described above (45-60 min. per lesson, quizzes, etc.), however you can also use it as a tool and make it serve you. The beauty of the books is that they are organized in sets, but you can use them in any order and any pace you wish, and buy them individually. The World of Plants has about 30 lessons, and you can definitely pick and choose which ones you do. Each one is pretty short, and there are a few discussion questions and an activity with each lesson, but the quizzes and tests are all separate so you can totally ignore them. I think it would make a good spine.
My daughter is actually doing this book right now, and she just reads the lesson, we do the questions orally and then she notebooks the lesson. If there’s an activity she wants to do, then we do that, too. It can be used in a very CM way, even though the description doesn’t sound like it!
Just wanted to clarify that, as we’ve used God’s Design lots in our homeschool and almost never as written!