ZenScanner,
Have you seen the
Beast Academy books? They're from Art of Problem Solving.
They go into depth and are heavy on teaching mental math and on showing patterns in mathematics. Right now they have books for what they call grade 3 -- but don't be fooled by that label. The books are sophisticated and go way, way past any third grade curriculum I've ever encountered.
For example, multiplication (Book 3B) starts with the basics and moves quickly to perfect squares. Then it goes into mental math for squaring numbers and the patterns that exist behind the numbers.
Honestly, I see these books as doing what EM and TERC are trying (but failing) to do. They're focused and move logically from one idea to the next and encourage in-depth understanding at every step of the way.
