Easiest way to make a custom distro?

You might consider using something like Archiso (https://wiki.archlinux.org/title/Archiso). On an arch-based distribution, you can try “mkarchiso -h” to get the options, and from there determine what you want.

I don't know what you're trying to do but the easiest thing would be a bash script you would run after installation which does everything you want to change if it's just some setting and packages

Doesn't Nix OS support this out of the box?

The big boys often start with Gentoo. Note that the Gentoo package manager can install precompiled binaries, even if the distro itself only uses it that way for a small number of packages. You just have to set it up appropriately. Might be more work than you want to do if your creation isn't as "original" as ChromeOS, though.

the best way i find is do a minimal install then if you are using deb bases system you just create a package.txt file with a list of all the packages you want to install then you just run sudo apt install < package.txt

Then create a config directory tree for all the home directory configs in the correct paths then you can gnu stow to create links to the correct path in the home directory.

Then you can commit it to a git Repo or copy it to a file share. so that on your next system install you just do a git clone to your stow directory then run stow to create the links. note you can also manage your package.txt in git as well.

You might not need to, If you just want a distro you can install with your dotfiles and programs maybe a setup script which you can run after installing a distro is a better idea.

I can't say if it's the easiest, but looks like using the archiso tool you can create pretty easily Arch-based system.

The harder part is using the Calamares installer.

Just search for "(your favorite distro name here)minimal install", download whatever pops up first, install it and customize it to your hearts content.

But if you are talking about (entirely) making a zero, brand-new distro... then Linux From Scratch is your best bet.

You’re probably looking for Linux From Scratch. Just so you know, it is quite a commitment. But if that’s your bag it’s also a lot of fun.

It's easy to make a Debian or Ubuntu preseed ISO with whatever customization you like.

For Ubuntu base check out Cubic. I had good results with it.