Is SM the best method for a girl approaching the end of Kindergarten?
It depends. I used SM with one of my kids who had suffered under dreadful math instruction. It was so bad, his skills regressed during the first semester of that year. SM, with its logical progression, got him on track and helped him a lot. That's a huge strength of SM.
Beast Academy from AoPS is also a very good and offbeat program. The comics and the friendly monsters make it appealing to kids. It also tends to explore a subject in more depth than other programs (including SM). The downside is that it's geared toward grade 2 and up (with books set to grade 3), but if your daughter's skills are near the end of grade 1, she'd likely benefit from it a year or so.
TBH, I've been teaching my kids since they were very little, and my approach has always been to use a combination of materials. No single source will provide everything you want, but it's not terribly difficult to cobble it all together from different sources. You can even create stuff yourself --- even if a self-made material is just an example or two that you think is missing from everything else you have. Because your daughter is so bright, you can also go way outside the curriculum, such as by showing her how to count in other number systems (e.g. hieroglyphics, which used base 10). If she can only read numbers in the hundreds now, just teach her how to count in hieroglyphics up to 999. Then add powers of ten as she's ready. This kind of stuff can be a lot of fun for kids. You can also do Roman numerals to whatever she's ready for. Contrasting both could lead to a discussion about how awkward it must have been for Romans to do arithmetic (how DID they multiply LXVIII times XXIV, anyway?? Even adding those numbers would have been a pain!) In contrast, arithmetic was far more straightforward for the Egyptians, even without a zero (which is another great topic for slightly older kids).
...would books or digital be a better way to go? My gut instinct says books, but maybe a digital approach would be more fun and less like additional school lessons. If using books, would the U.S. Standards or CC edition be best for supplementation?
Personally, I stick with books, and we didn't use the CC edition of SM. SM works well already, so why use an edition geared to a program (well, a non-program) that has problems?
Finally (and perhaps most importantly), should I supplement at my daughter’s intellectual level, or follow along at the pace of her classroom lessons? I gave my daughter the SM placement tests and she breezed through 1A and over half of 1B before she started to run into problems. In addition, her teacher said she’s reading and writing at an end-of-first grade level. At the moment my daughter seems quite happy at school, and isn’t ready socially to skip a grade, so I’m a bit hesitant to do anything that would increase the odds of her becoming bored in class.
IMO, this is more of a philosophical question. Here are some observations I've made with my kids (ages 11 to 16).
*Moving ahead of an age-grade curriculum allows the child to have work that's appropriate to her level, and will require her to struggle for understanding from time to time (with my kids, this is what I like to see).
*Yes, she'll be ahead, and that's hard to deal with in many ways. The question to answer is: Which option do you like less: being ahead and the awkwardness that goes with it, or being bored/consistently underchallenged?
*IMO, HG+ kids will be bored in math and LA-type classes anyway, unless they're at a school that's been tailored to HG+ kids (see next point).
*Some schools may let the child work with you at home and do your assignments in math class. All my kids have benefited from this approach.
*I strongly recommend working in a methodical way. Given your comments about her classroom environment, do you think that your daughter will get a solid grounding in mathematics at her school? If not, you're probably the best person to provide it.
