Grade 6 math

I wouldn’t suggest changing from what’s working.  It sounds like you just need to invest in a timer and a bookmark.  Don’t worry about finishing the book in a year.  They need to be ready for Algebra 1 by 9th grade.  If they understand the concept, move on and skip some review.  If they don’t understand, stay there until they get it.  You can’t rush their understanding, but you can lose them in long lessons.  I think any math curriculum can get long.  Switching math can be detrimental because of a different scope, sequence, and method.  (Don’t ask how I know.)

To add in CM math methods, you can buy the math guide sold here written by Richele Baburina.  With it, you can learn how to use a variety of household items for math manipulatives.  Supplement with oral math story problems like those found in Strayer Upton.  Be sure they tell you “why” after they answer.  The video does a great job of showing sample lessons at various levels.  Maybe you could switch over your younger children, but supplement with Singapore for your older children.