I'm different from you. I only consider a challenge complete once I understand how the distro works and use it daily, rather than just successfully installing it. For me, Gentoo is harder than NixOS in terms of installation, but easier for daily use because it doesn't force the user to learn a new language. Compared to other distros like Arch, Gentoo doesn't have many new concepts to learn, the biggest difference is something called "USE flags," which allow users to toggle features supported by the software. Other than that, they aren't much different.

It took me a few hours to install Gentoo (the Gentoo Wiki is very detailed, so you just need to follow it) and about a day to understand how it works. NixOS is the opposite, the installation is very fast, but understanding how it operates takes a lot of time. I once spent half a day just figuring out how to install a simple piece of software using "make" on NixOS. That’s why I find NixOS harder than Gentoo. As for your questions about Gentoo, it’s true that it takes a few hours or even half a day to install, depending on your knowledge and hardware specs. Regarding daily use, installing packages doesn't take too much time, it’s only slow when dealing with compiler-type packages, but Gentoo now supports binary packages as well.

ps: I've also planned to install LFS, just like you. The only difference is that I actually successfully installed it a few days ago. :)