Arch is aimed at people who know their shit so they can build their own distro based on how they imagine their distro to be. It is not a good distro for beginners and non power users, no matter how often you try to make your own repository, and how many GUI installers you make for it. There's a good reason why there is no GUI installer in arch (aside from being able to load it into ram). That being that to use Arch, you need to have a basic understanding of the terminal. It is in no way hard to boot arch and type in archinstall. However, if you don't even know how to do that, your experience in whatever distro, no matter how arch based it is or not, will only last until you have a dependency error or some utter and total Arch bullshit® happens on your system and you have to run to the forums because you don't understand how a wiki works.
You want a bleeding edge distro? Use goddamn Opensuse Tumbleweed for all I care, it is on par with arch, and it has none of the arch stuff.
You have this one package that is only available on arch repos? Use goddamn flatpak and stop crying about flatpak being bloated, you probably don't even know what bloat means if you can't set up arch. And no, it dosent run worse. Those 0,0001 seconds don't matter.
You really want arch so you can be cool? Read the goddamn 50 page install guide and set it up, then we'll talk about those arch forks.
(Also, most arch forks that don't use arch repos break the aur, so you don't even have the one thing you want from arch)
I'd say they are similar, but they have somewhat different philosophies. Slackware maintains a KISS philosophy and comes as a full system. It has its ncurses interface for installation. Some might find that helpful. Arch is purely CLI, so you need to know the commands (or write them down) to set the keyboard layout, set up a network connection, time/date, and so on. You build your Arch system from the ground up, but Pacman handles dependencies for you. Slackware comes as a full or minimal installation (or you can choose individual packages at the risk of breaking dependencies). Slackpkg does not handle dependencies for you. Both will require you to have some sense of what's going on under the hood. You'll need to edit config files and be a sysadmin of your own system.