Various ways of incentivizing learning in kids have been tried. The only one that shows long term benefits was, iirc, paying them to read a book.
Math skills degrade rapidly with disuse – ask any new college grad who took calculus freshman year to find a derivative and unless they continued in math classes, they probably can’t.
But reading skills improve with practice and then don’t degrade as much, since we’re constantly reading stuff anyway. If your kid “gits gud” at reading early on in life, it’s basically an easy button for the rest of their academic career, and as a nice side effect, they generally get pretty good at formal writing through osmosis.