@Local Minimum: Great suggestions. I was thinking of dropping the shapes around the numbers. I originally made them to help with color-blindness, but then I dropped that, yet left in the shapes.
Also I think the equation templates, which I thought would help players in the first levels, actually confuse them. I'll keep thinking how to improve them.
Once you make it past the first few (hard-coded) levels, the rest are procedural, and probably easier, at least for a while. I'll have to figure out an even slower progression through the various skills (subtract, then multiply, then add/subtract, etc.).
I could score it by equation rather than level. Up for consideration.