What's the difference between Grade 2, Grade 8.8, and Grade 10.9/12.9 in these kits?

It's a hardness and tensile-strength rating. Grade 2 is mild steel and fine for cabinetry, electronics, and 3D printing. Grade 8.8 is the metric equivalent of US Grade 5, common for automotive use. Grade 10.9 and 12.9 are higher-tensile alloy steel for high-load applications. Several listings here call out 10.9 or 12.9 alloy steel — that's the spec to verify if you're using these on anything stressed.