I'm thinking of moving away from Mint when I'll be building up a new PC in the coming year. Nothing wrong with Mint, but Cinnamon has a few minor annoyances (though again nothing major). After looking at a few DEs, I really loved KDE, (with XFCE and LXDE/Qt being second). And sure, I can install KDE on Mint, or use its XFCE edition, but why do it, when I can use a distro that uses KDE natively?

So... this is what I'm looking for:
Something stable. No rolling realeases; LTS only. I know this is a personal preference, but I would like my OS to remain same and be supported for a few years before switching to a new version.
Relatively good support for GOG games. I know only Ubuntu is officially supported, but from what I read there usually aren't any major problems installing them on other distros?
Being actively worked on, with a decent wiki/forums to get help from. Doesn't have to be super popular, but I don't won't it to be someon'e hobby project where you if you post a question on the forums you have to wait for a week+ till one one the three people who use it replies to you.

From the research I did it seems my best options are Kubuntu (though I'm not a fan of Canonical, but it does offer long LTS), OpenSUSE Leap (though I'd prefer to have some proprietary software included in the installation; there is also GeckoLinux based off it that does include them https://geckolinux.github.io) and Mageia (but each release version is only supported for 18 months or so).

Does anyone have any comments/recommendations on either of those? Any other distros you'd recommend?

I'd say the safest route would be Kubuntu. When it comes to KDE with LTS, Kubuntu comes up as the most recommended, seeing as it has a solid base.
Opinions will vary, as expected, but when it comes to KDE, Neon and Kubuntu are some of the best choices. Neon being KDE through and through, but somewhat peculiar since it mixes the LTS aspect with the novelty of the environment itself, so more of a hybrid. No wonder, considering the team behind it.
Kubuntu feeds on the popularity of Ubuntu and its widespread support. Solid choice all-around.
By and large, those are the distros that do KDE best.
Not going to mention others since you asked for stability.

No rolling releases? But then you describe why rolling releases are great? They are supported indefinitely (so long as you don't let it get super behind in updates). You never "switch to a new version" in rolling release. It just updates all the components as they go.

I never suggest LTS. and I'd never pick a distro on Desktop alone; virtually all of them, (even red headed stepchildren like BSD) should be able to install KDE in some form or another, even if they are starting with some other desktop.

Fedora has a 6 month release cycle, and a support cycle of a couple of years. Fedora 32 will be EOL this May, while Fedora 34 will become the mainline release in about a week. The centralized COPR is far more sane than relying on Uncle Shady's PPAs, and the RPMfusion repository covers your worries for nonfree/proprietary software.

The upgrade process is surprisingly sane, and updates themselves can be nearly completely automated.

LOL

But... but... that's the best part. Every 3 years or so I back up all my game saves (my personal stuff are on a separate disk alread), format my main disk and install a new OS from scratch. I make a day of it and look forward to it for weeks.
It's just like Christmas and getting a new present. Except less frequent.

We are very different people :)

I've been running the same install of Arch since 2012. It's on its third system now. I love not re-installing.

Thanks for your input. I did look at Neon, but I think I'd prefer the stable version of KDE. ...
huh?
I'm not using Windows. Doesn't Fedora have a support cycle of 13 months only?

It's why I used the term hybrid. Neon is a trade-off. The latest software, but also instability. A testing ground of sorts for the KDE team.

The video was mostly meant for Orkhepaj.

Ah, haha. Sorry then. I have her on jerkmuter and didn't notice that post#2 is missing.

Video's funny.

With just one system, sure, it's Christmas. If you have to reinstall Linux on 10+...

Then again I also get your point, because even though Linux does not collect the bloat Windows does, it is nice to fully purge it with a base reinstall every 2-3 years or so. Keeps you on your toes on what you need to do to setup/configure your system too.

I have no recommendations, since you're very specific in your demands. Kubuntu seems the way to go if you want something LTS with KDE.

P.S.: I've never liked Cinnamon, but I find the Mate flavor of Mint to be my ideal OS at the moment. Call me old fashioned.

Get off my lawn :-D
Isn't Cinnamon GNOME 3 under the hood? I'd rather choose the base and start from there. Up until recently I disliked the general feel of GNOME, but nowadays I quite like it. At times more than KDE.

That was pretty insightful. And here I was, wonder, why since last year, Windows 10 exploded in Steam charts. Was the pandemic situation or Windows 7 end of support.
Nop, actually Microsoft shove the upgrade in users computers, being so obnoxiously annoying that most people just upgrade to get rid of the pop-ups. Well played, it worked.

The only Distro's I've tried with Kde were Kubuntu (5+ years ago) and Manjaro very briefly, so no help here.

It certainly did.
Why MICROSOFT Force Installs Software Without Permission

Here's the issue though: no matter how much you try to debloat and secure Windows 10, there is always a bump in the road. That's how the system itself is built.