There is now an interesting guide by @mid_kid (https://mid-kid.root.sx/git/mid-kid/bootstrap/src/branch/master/gentoo-2024/gentoo.txt) on installing #Gentoo from just source and a tiny 200 byte kernel. At the moment it's a bit longish and starts with #livebootstrap then pivots to #LFS to obtain 64-bit toolchain and finally bootstraps #Gentoo from there.

Potentially some steps could be optimized, and #LFS removed but it works.