Not often I meet another herbsluft user in the wild! waves

Good to hear that you’re slack-curious! :D Gentoo is a fantastic distro, so great choice! I run Gentoo on my second computer. I’ve always loved it, but Slackware was my first linux experience, so it has special meaning to me. Maybe try Slackware in a VM? You’ll be compiling a lot from source on Slackware too if you need stuff that’s not included in the base system, but without portage for deps management it’s a lot more cumbersome. You can of course use sboui, slpkg or some other tool that can manage deps, or use flatpaks, appimages, distrobox or whatever to keep your base system clean.

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.

That depends on what the beginner’s goal is. Arch could very well be a nice beginner distro, as could Gentoo or Slackware or any other “hard” distro if you’re determined to learn. My baptism of fire was on Slackware in the 90s (which I’m still on), long before “beginner distros”. Trying and failing was a big part of the fun. If you’re determined to learn, I don’t see any issue with starting with a distro that doesn’t hold your hand.