Pop!_OS’ Cosmic DE is a recent fork of Gnome. It’s the fourth “No, I’d rather put up with being a FOSS project manager for the rest of my life than keep using Gnome” DE I’m aware of, after:

• MATE is basically “No, we like how Gnome 2 was, so we’re keeping it.”
• Cinnamon is basically "GTK3 is cool but we’re not doing all this “almost nothing works because you HAVE to use it EXACTLY like this” crap. Linux Mint is basically a grievance distro; it’s defuckulated Ubuntu with defuckulated Gnome.
• Unity. Because Canonical was reinventing the wheel, the inclined plane, the lever, human speech, electromagnetism, fire, antiperspirant, package management, the display service and the init system, so why not the DE as well?
• Cosmic. I’m not sure why System76 bothered, other than Gnome wasn’t sparkly enough.

Onto your actual questions:

1. I don’t know if Manjaro is European or not, but I have given up on them. They’ve made a lot of rookie mistakes like letting security certificates expire, they dropped the ball pretty hard with the PinePhone IIRC, and every time I’ve attempted to use Manjaro it just didn’t work that well.

Pop!_OS is the in-house distro of System76, a for-profit computer vendor based in Colorado. It’s meant to work well on their hardware out of the box; the big claim to fame that I saw when they were the trendy distro of the week is they had a version that had Nvidia drivers baked into the install, so to the end user “If you have an Nvidia card, click the link on the website that says Nvidia and it works.”

Linux Mint, as previously mentioned, is mostly concerned with distancing itself from questionable decisions made by Ubuntu and Gnome. It maintains compatibility with Ubuntu, so Ubuntu install instructions for software pretty much always work with Mint, and yet the DE is more easily understood by Windows users.

Fedora is Decommercialized Red Hat, honestly I don’t really see them making much effort towards beginners besides maybe their immutable/atomic versions.

Customization of KDE is going to be pretty similar if you stick to things you can change via the settings menu, at some point during “customization” you’ll find the boundary between DE and distro. Yes, many people use KDE. Unlike Gnome, they haven’t pissed off four different groups over the ages resulting in forks. KDE is mighty popular.

“Is it really beginner friendly, snappy, stable?” Lots of things “it” could be, if you mean KDE…yeah? I personally prefer Cinnamon as a “give to grandma” OS because KDE tends to slop more crap on the screen, especially in that One Settings Menu To Rule Them All they have. This is a subjective experience but, if I ever found myself thinking “Is there a setting to change this?” I felt more able to find that setting in Cinnamon than in KDE.

2. I know very little about OpenSuSe, I’ve never once tried a SuSe-based distro.

3. Out of the three you listed, I’d go with Fedora. I’m typing this on a machine running Fedora KDE. I would personally recommend KDE Neon or Kubuntu before Manjaro or OpenSuSe. Manjaro because I’ve had bad luck with Manjaro, and I have no experience with SuSe. I might also steer you to EndeavourOS or vanilla Arch over Manjaro.

You’ve used the word “stable” a lot. Users typically mean “is it going to crash?” None of these are going to crash, you’re not going to crash Linux. What you might find, depending on the distro, is someone will push an update to a software package that breaks your workflow. Debian Stable, Ubuntu LTS, distros like those do point-in-time feature freezes to avoid that possibility. You might not get the newest features but your system will continue to work like it always has. Rolling release distros like Arch push updates as soon as their developers release them, so you get all the cutting edge features, bugs and breakages.

Of the three you listed, Manjaro will be the least “stable” by that definition; it’s a fork of Arch. Fedora and OpenSuSe are both forks of commercial distros so you get that less, but I can attest on Fedora it does happen, they broke the lock screen last month. You want to never reboot your computer to find something suddenly doesn’t work, go with Debian Stable.