A lot depends on your child's current skill levels, your goal, and your child.
Writing With Skill is the follow on to Writing With Ease.
Building Christian English 7th grade is doable by a 9th grader who hasn't had that kind of intensity. So 7th, 8th, and then their two book follow up, "Communicating Effectively" (CE Book 1 and Book 2 can be done in either order, they treat different topics).
I feel I've heard good things about Writing Strands but don't have a copy of the program.
If you search for Cathy Duffy Reviews, she lists a *lot* of programs under composition/grammar.
Note that if you are supplementing/enriching for someone that age group, you may be stuck with more narrowly focused single-year programs more likely to meet specific interests, because it gets harder and harder to push them to do work outside of class. Something like the Pirates' Guide t' the Grammar of Story, for example, which has a pretty fun schtick to it - creative writing, as taught by a pirate.