TBH, you don’t really dip your toes by just switching distros. If Ubuntu works well for you, I see no reason to switch (even if I personally have a beef with it). I hate ditro-hopping personally, I don’t really care what others do.
Being at the beginning, I’d recommend Fedora and staying away from Manjaro. I have had Manjaro installs malfunction in unexpected ways more than Arch installs. Sure, YMMV, but from my experience and from other online complaints, I’d say stay away from it.
You can try Arch if you want to see how the Linux desktop works at a high level (services, DE, display servers, login managers etc.), following the Arch wiki is not hard if you have patience to read everything and not between the lines. If you want Arch, but with an easier installer, try EndeavourOS, it’s basically the spiritual successor to Antergos (just “Arch with a GUI”), but it has a closer base to Arch than Antergos did.
But again, your desktop needs to be functional, so distro-hopping and testing stuff on your daily driver is… not recommended to say the least. Stick with Ubuntu or Fedora and if you really want to learn, try installing Arch in a VM, poke some AUR packages, let them compile and after you get used to it, dip your toes into either Gentoo or LFS (Linux from Scratch). Or, you could try to see alternative inits and service managers, but still keep the (now familiar) Arch base, by using Artix first (I’d recommend OpenRC, because that’s the default and it’s also Gentoo’s default).
At some point, when I stop using low-end hardware, I’ll probably try both Gentoo and LFS. I most likely won’t daily drive them, I’ll keep them in VMs or LXC containers (well, at least the former), but they are on my list of things to try out. I’m kind of impressed by myself, that in 10+ years of using Linux, I used to distro-hop a lot in the early days (mostly between Ubuntu, its flavors, Arch and its forks), but with my growing distaste for distro-hopping, I managed to never even touch a Gentoo iso. Well, better late than never.