I suggest LFS if you want to learn the complexity of creating a distro from scratch. (You might not succeed on the first try. I gave up multiple times before forcing myself to finish it) I never tried Gentoo so I can't really say anything good or bad about it. My question is: what are you expecting to learn from this? you say "learn more" but what exactly do you want to learn? Because if it's "becoming better at linux" you can definitely do it with just Arch. If you just want something "more difficult" to install, I guess you can do it.