Linux

My big question would be why are you starting with a dual boot? I would recommend trying each one with a liveUSB or in a virtual machine and simply do a single boot with the one you like better. There’s likely little need for you to actually maintain two distros unless you have a very niche use case that one distro can’t solve.

My advice would be to just relax and realize that the underlying OS is 90% the same regardless of what distro you choose. All the discussion you see on different distros, package managers, snaps, wayland, etc. are all the other 10%. It really doesn’t matter what distro you start on as long as it’s a general purpose distro (both of the ones in your OP are): once you learn the first 90% of linux, you’ll develop your own tastes, and then you’ll be able to decide on the remaining 10%.

It's a bit of an unpopular opinion, but if you pick a mainstream distro there isn't a lot of difference between them. Especially to somebody who is new to linux.

With most any distro you can use KDE, gnome and other desktop environments. You can pick which one you want to use when you login. So don't think you're tying yourself to KDE if you install kubuntu or something.

If you want an easy way to switch to a new distro make sure you create a separate partition for /home. Then if/when you want to install something new you can have it overwrite everything except your home directory. So all your steam configs and games will be left untouched (for example). Alternatively just backup /home somewhere and restore as you need.

For Nvidia I would recommend Pop!_OS since it makes things really easy. Either that or Debian with KDE. More mainstream = more users = better support.

OpenSUSE Tumbleweed is an awesome distro with up to date packages. KDE is also a great choice, especially with Plasma 6.0 around the corner.

I wouldn't worry too much about Nvidia drivers, just follow OpenSUSE's guide [1]. The remaining issues of Nvidia with KDE Wayland are getting fixed over the coming months.

Edit: OpenSUSE can't ship some codecs by default for legal reasons (like RedHat, Fedora), but makes it simple to enable them (optionally through graphical YaST) [2].

[1] https://en.opensuse.org/SDB:NVIDIA_drivers

[2] https://en.opensuse.org/SDB:Installing_codecs_from_Packman_repositories

Keep notes on what you do including outcomes. You can always reinstall, skip all of the extra crap you didn't need to mess around with and have a good clean system without having to back track.

I'm also a beginner and I noticed that debian let me install a lot of possible desktops. So I installed all of them, and I can switch on the password screen which one to open. I'm trying some stuff this way.

OpenSUSE + KDE is a really solid choice, I'd say.

The most important Linux advice I have is this: Linux isn't Windows. Don't expect things to works the same.
Don't try too hard to re-configure things that don't match the way things are on Windows. If there isn't an easy way to get a certain behavior, there's probably a reason for it.

If you're going to be using a DE and mostly do stuff through the GUI instead of terminal/command-line then make sure you can go admin mode (Root/Sudo).

Besides small annoyances I had with KDE Plasma 5's UX the main reason I didn't like it was that often enough I would have to use admin privileges but I couldn't do it through the GUI File Manager (Dolphin) so I frequently had to use the terminal.

It should be possible to have admin privileges in Dolphin but I was a noob and didn't know how (and still don't even now).

If you end up facing that issue then either be a bit smarter than me and look up how to do that or use Nemo, another file manager, which is more or less the same thing as Dolphin except when I ended up using it on Linux Mint a while back it let me use it as Root as a feature out of the box.

And for the record I don't like Linux Mint, apt package manager sucks (package managers are basically app stores where you get all your stuff), but at least it was super easy to install and Nemo was a good file manager.

If you don't mind tinkering and have a secondary device with an internet connection in case you break something then I would recommend Arch Linux. Or you could try it in a Virtual Machine I guess.

Pacman (Arch's package manager) is a hundred times better than Apt, and then there's the AUR on top.

Also while I've never used it I hear a lot of good things about EndeavorOS, Arch Linux but supposedly easier

If you run into any issues or if you're not sure how to do something, feel free to ask.
I'll do my best to point you in the right direction.

I’d rather avoid a Linux distro that’s implementing something like ads or telemetry…if that’s even a thing that’s happening?

Fedora has some telemetry, but as far as I know, you can turn it off during the installation. Some desktop environments like KDE Plasma also have options for telemetry but it's disabled by default. If you want it, you manually have to enable it in the settings.

Should I avoid OpenSUSE

Some software might not be available on openSUSE

or KDE Plasma

Not really, Plasma has a shitload of features and customization options, if many options tend to overwhelm you, you might be better of with a different desktop environment, if you are fine with customization options, Plasma is great for you.

Are there any ‘10 things to do first when installing Linux for the first time’ recommendations?

Since you said that you want to install openSUSE, I recommend this video: https://youtube.com/watch?v=ajVqJ1nl9bM

If you have a spare solid state drive or a portable HDD (most USB sticks are a bit too slow for a proper install), maybe install a Linux distribution onto that portable device, then you can just boot from that, then you only need to boot switch in Bios to Windows when you need to use it again. This has the advantage of you not needing to setup a complicated dual boot setup. When you are settled in with Linux, open your drive where Windows is located from Linux, copy your files over (don't forget your bookmarks and saved passwords) and then you can clone your linux install from your portable drive over your old Windows install using a tool such as dd.

I used to run Tumbleweed with KDE on my Nvidia system. I found the rolling release structure of Tumbleweed to cause extra work for me, because kernel updates came frequently and occasionally broke the Nvidia drivers. As a workaround, I ended up pinning my kernel to an old version.

Nvidia drivers have been at least a little troublesome on every distro I've used, particularly with the additional CUDA libraries.

One nice thing about Suse is that it uses BTRFS by default, and you can use snapper to revert your whole system if something goes wrong. So if Nvidia shits the the bed after an update, it's easy to roll back. Most distros default to ext4 and do not have snapshot support by default, which feels like living in the stone age to me after using Suse and BTRFS.

Of course you CAN set up BTRFS and snapshots in any distro, but that's a lot to ask for a beginner with Linux. I strongly recommend choosing a distro that does that for you, like Suse.