Honestly, just test out a bunch of different distros over the course of a couple of weeks (or months even). As for which distros are worth considering for 'playtesting', that's entirely reliant on your personal 'Linux Journey' and whatever you find interesting. Though, if I'd be forced to make a list, then it would consist of the following:

1. Start off with Arch, Fedora, Linux Mint, openSUSE Tumbleweed and Ubuntu. (Debian is absent from this list because you're already using it.) You might even combine this with using different desktop environments on each; as this might have more influence on the experience than the distro itself.
2. After indulging with the 'veteran-distros', there are some different directions you could go from there. Perhaps you could try a distro that
   • doesn't use systemd; the likes of Alpine, Artix, Devuan, Gentoo, Guix and Void come to mind.
   • is 'immutable'; the likes of blendOS, EndlessOS, Fedora Silverblue/Kinoite/Sericea, Guix System, Nitrux, NixOS, openSUSE Aeon/Kalpa, Vanilla OS and Ubuntu Core Desktop come to mind.
   • takes security and/or privacy very seriously; the likes of Kicksecure, (openBSD, QubesOS (even though neither are Linux distros),) Tails and Whonix come to mind.
   • holds a conservative stance in regards to software and doesn't like to enforce change to its users. This enables you to learn the intricacies of its system once without ever having to forego that knowledge as times passes; Slackware.
   • is unrivaled in terms of freedom it allows its users; Gentoo.
   • unshackles itself from GNU; Chimera Linux and (to a lesser extent) Alpine comes to mind.