I have used Singapore Math for 6 years now - Kindergarten through 5th grade. I have enjoyed using it but now I am moving on with my older child.
We are beginning to incorporate Beast Academy as a supplement, with the plan of completing Beast levels 3-5 through 6th grade. After that I plan to move to Art of Problem Solving. See my review of Beast Academy here.
While Singapore Math does have a 6th grade level, my research told me that it was more of a comprehensive review, which would be in line with what they would be doing in Singapore and other Asian countries - preparing for comprehensive exams in 7th grade.
After Singapore Math level 6, historically your only option was to them move on to New Elementary Math, which was a decided jump in difficulty from the primary math. This also makes sense because at that point, everyone who is continuing at an academic level in Singapore and other Asian countries is now moved on to secondary level math.
They also have teachers who are trained to teach that level of math so the teachers guides and answers were pretty sparse if you ordered New Elementary. The final thing that turned me off to Singapore's New Elementary Math was the fact that it is integrated math, meaning they don't follow the same sequence of discrete maths that we do in the U.S., i.e. pre-algebra, algebra 1, Geometry, Algebra 2, etc. Instead, New Elementary just 4 years of math where its all combined. I didn't think that using an integrated math approach would be good for someone in the U.S. system if there was a likelihood that additional math would have to be incorporated in high school.
Singapore math also offers Discovery Math. After looking at that I knew it wasn't for me because it felt very much like it was intended for a school setting and is decidedly common core aligned.
To be sure, I still 100% endorse Singapore Primary Math and am still using it. If you are interested in purchasing it, I suggest you take a look at my Singapore Math Review. It's long but very informative.