Well, no experience with in-class differentiation, but my kids have not been particularly capable writers for their ages (to say the least!) and they've been able to handle and benefit from the written demands of Singapore Math 2A/2B as five-year-olds. They did get me to scribe occasional bits: writing number-words into the hundreds, for instance. And they only wrote the numerical answers to word problems, never answering in full sentences. But the curriculum doesn't get fussy about that anyway. Singapore is a little more advanced than a typical North American curriculum, so that's probably up to an early 3rd grade level.
I also found that my kids needed very little instruction, at least until the 3A level and beyond. Maybe a 30-second explanation of how an exercise or algorithm works, sometimes 5 minutes working through an example together, but often they would be days or weeks without needing any help except for occasional answer-checking. (Many of the workbook exercises are self-checking puzzles and such, so there isn't even a lot of adult checking needed.)
You can look over the program at www.singaporemath.com . They have page scans of the various books, and placement tests. Primary Math 3rd Edition is the most popular version of the program, the one most people go to. It's inexpensive, particularly when compared to most on-line services. No idea how much ST Math would cost, but I bet it isn't cheap.
Edited to add: Here's a page scan from early in the 2A book, to show the writing demands, and how my newly-5-year-old managed it.