I have heard of Nand2tetris, but I always thought it fell more into the 'read a textbook' bucket than the 'do your own technically challenging project to reinforce the learning' one. But I guess it's a project-based textbook, which kind of blurs the lines.

Thanks for sharing those examples. I guess the common thread here is to work on _building_ things that exist at lower levels of abstraction, rather than just knowing how to use them as a black box. I guess most project ideas that satisfy that criteria would teach some fundamental skills too.