It is hard to figure out, isn't it, especially when you're switching to a different way of teaching. I can share with you what we do here, if that might help. The older boys (ages 11-14) each have a literature book I've assigned them at all times. They read about a chapter per day, unless the chapters are extraordinarily long or short, and must give me an oral narration from that book each day. They are also reading living books for science daily, and after each reading, another narration, and the same for their daily independent history reading. I haven't usually read all these books, so I sit with the book opened to the section they're narrating, and skim through as they tell me about it, to make sure they've gotten the main points. At first, this took a ton of work for them and for me! I know CM said one thorough reading and they shouldn't reread, and that was our goal, but at first, they really hadn't developed that skill, so I had to teach them to break it into paragraphs if need be, and make sure they understood the main concept in each.
Now, when I listen to narrations, I'm not mentally checking off to make sure they've gotten certain main ideas, because each child will focus in on different things, some of them things I'd not noticed and connections I hadn't made myself. I do listen to tons of narrations every day, but doing this all year (this was our first year doing narrations across the board), I can see how much they're learning! Once a week, the 11-year-olds will do a written narration of their history reading, and the 14-year-old has more writing assignments.
I agree with another poster who recommended breaking up the tasks with expectations and goals, for things like the memorization. I have a really hard time sticking to that myself, so I know that doing things regularly really helps me. For example, if we do Bible memorization or whatever other memory thing we're working on every day at snack time, that helps a ton. Perhaps there is once a day you can schedule the grammar work, at least until he's in a groove with learning it, and then schedule and check in once a week for that particular skill?
You aren't setting yourself up for failure if you use CM, because really, the lady required a lot of her students! And so it's up to us to figure out what that means for each of our kiddos, schedule it, and require it. I am hard on them I suppose, in the read-a-book front, but it's school, right? And they do come to enjoy it even if not all are dying to pick up a book for fun (but some are!).
Prayers for peace and direction for you as you decide what direction to go in. I can't speak for everyone else, but I know I'm still figuring it out as I go along as well!