When we homeschooled that’s how we did it, too.
We used Saxon math and did math year round. The first 30 or so lessons in Saxon are review, so when we’d start a new Saxon book, we’d take the tests, not do the lessons, up to the point where questions were missed on a test. That’s how we determined where/when to start the lessons in that particular level.
What is Saxon Math?
We used Singapore Math
When we used Saxon, we did it a little different. All I made the kids do was the practice part of the lesson for those first 30 or so until they either got to new material or, like you, started missing problems.
Even after that, if we came across a chapter they had down cold, we’d skip it and go on to the next.