RSM and AoPS are great for a mathy kid, but neither are near SS.
Although I've always heard good things about RSM, I'm most familiar with AoPS.
If you think your kid would like it, AoPS has online classes through something like zoom via the AoPS Academy. The classes are aligned with BA up to 5th grade. The regular AoPS Online classes are less expensive and are through text-based chat, so they're not a great choice for younger kids at least in my opinion.