Review Supplement with MUS

We have been using MUS for many years and we like the way it teaches, however, my kids have trouble retaining. For this reason we have dabbled in other curriculum (enough dabbling to make us 2 years “behind”). I really like the variety of problems in Saxon and so did my son – we only did evens or odds each lesson, but I am not happy with the teaching part. It seems strong on the how, not so much on the why. My son seems fine with that, but it bugs me. So I am wondering how I can use MUS as my main math, but add some review to it. Has anyone tried this? Or maybe I should spend more time on the review pages and less time on the new concept pages. Any words of wisdom?

On a separate subject, he is starting high school next year and still doing elementary math. Not because of his ability, because of curriculum hopping. Does anyone know of a good one year teaching/review of all elementary concepts book? I have been considering Masterbooks Principles of Math, it is two books but has a one year schedule also.  It looks very wordy though. Any other ideas? At regular pace with MUS we wont hit Algebra till 11th grade – yikes.