Back in college we had a project that was this. Most of us used Linux From Scratch, a few of us used gentoo, having played with arch since, you could use that too.

https://www.linuxfromscratch.org/

Penguins Eggs is another way to do it:

1. take an existing distro
2. customize it how you want
3. save it as a 'new' distro with Penguins Eggs

To me, ublue is the easiest way of creating and maintaining a custom linux distribution

https://github.com/ublue-os

LFS (Linux From Scratch), though building a custom NetBSD or OpenBSD Kernel and making an ISO with custom packages/configuration is a lot easier.

You're speaking in the past tense. There are at least two more joke distros around which are active as of now: Justin Bieber Linux ('Biebian') and Rebecca Black OS...

Hannah Montana Linux is just a joke?

But that was what I was going to use once Windows 10 dies... 😩

A themed Nix setup would be great for this