Oh, sorry, i didn't read all the way to the end on the description (just enough to understand how to move the shadows ;). I'm not well-versed in anything web, so sadly can't help you on the js to WASM part :(
If you like those kind of mechanics/procedural in-game generation, L-System are definitely worth checking out (you're already using them if i understood your algo correctly), they're used to generate 'natural-looking' trees, cities, landscapes, etc. in games and CGI movies (I spent some time on a personal project where i used them to 'grow' a digital garden (never got to the point of user interaction though)).