You could try Polya’s How to Solve It, and follow up with Schoenfeld’s Mathematical Problem Solving (which cites a lot of other material). These are rather focused on problem solving per se, and don’t really discuss larger-scale strategies for research or learning. You can try just diving into child development and education literature. Not sure what out there really tries to be comprehensive in summarizing the best techniques for teaching and learning arbitrary meta-skills.
You know what helps kids (and adults) learn math? The abacus/soroban. Yes, automaticity with math facts/basic math is important but what's really important is being able to represent the base-10 system mentally.
Students are actually taught how to think of numbers in groups of tens, fives, ones in Common Core math -- however, most are not given the abacus as a tool/manipulative.
I'm surprised that none of the postcard bots (yours included) have targeted the Christmas/holiday card or wedding gift thank you market. None of them seem to offer bulk discounts or options for uploading a csv or better yet integration with iOS or Gmail contacts.
Average number of wedding guests in America is 150, iirc.
