I am trying to help my sister plan for my niece for next year. I haven't been able to convert her to HOD yet, but I am working on her!

My sister has been floating in and out of Singapore math for several years and believes that by the start of next year, she will be completed with Singapore 6B. I think I remember somewhere on this board reading that after level 6, Singapore began to be hard to follow. I can't seem to find anything like that in my searches of the boards, but I don't really have time to search through all of the posts relating to math.
So, I thought I would ask for your thoughts directly. What do you use/plan to use after completing Singapore?