We went from SM 5 to Discovering Math/Dimensions Math with our first, and from 6 to same with the next child, each at about age 9. It's the middle school series from SM. Four courses of integrated pre-algebra, algebra, geometry, and beginning geometry. We've found it quite good, but not realistic for independent work. Parental support was definitely required at the younger ages, though after about 11 or 12, it started to become easier.

...pronounced like the long vowel and first letter of the alphabet...