I am in EXACTLY your shoes! I did Singapore standards grade 1 and we are in 2a and I am feeling like it didn't go as well as I would have liked. I even stopped the program for a few months last year JUST to focus on math facts, and while he is getting it slowly, I am starting to feel he has holes in his learning. We also are skipping the mental math, but that is because my son does better with vertical math then horizontal. We have also not liked the way everything is being taught.
We are going to go to Shiller Math. It is a large cost up front, but if it works, we can use it for his younger brother too. My son gets bored with the long units in Singapore, so I am hoping the spiraling will be better for him. Also my son has MASSIVE test anxiety (something I know he will have to get over, but at 6 I am not worried yet). The assessments are not set up to be like a test, they are to find holes and fix them before moving on. The criticisms I have found on the program is it jumps around a lot, and if you put your child in pubic school they may be far above their peers. Neither is a negative for me.
Don't know if this helped but thought I would try. 😊