I’d just start from very simple kernel and static init, and work my way up to adding more functionality. I’d use kvm with rootfs on p9fs - that allows playing with it without having to build images. I can throw together the initial invocation, if you’re interested.

Then start building simple core elements in a language allowing easy static linking - I’d use C with dietlibc or go. Start adding core userland programs, explore initramfs (without using something like dracut), add dynamic libraries and explore the dynamic linker, … - if you’re interested we could set up a matrix channel for questions (typically with some lag, though), and do a github repo to follow along.

LFS iirc goes for full desktop - the high level userland is very complex, but easy to understand when you know the basics. You pretty much learn how to compile lots of libraries - which has limited use. A full LFS style desktop I’d no longer recommend nowadays - it’s just too many dependencies to deal with. I used to build my own system (not following LFS) until the Xorg fork made it sigificantly more complicated - and things got just worse since then, and I never was using a complicated UI stack.

edit: I had a few minutes, so I’ve thrown this together https://github.com/bwachter/lll - you should easily get a kernel with a custom init running, and have enough to start experimenting. If you or anyone else is interested to go deeper I’ll set up a matrix channel for guidance.