Linux

Check compatibility for all your programs before you move. Most Linux programs work on Windows but not vice versa. If you're not in a rush, try switching to programs that have a Linux equivalent before you move so that you'll have less of a culture shock. If you need any killer apps that don't have a Linux equivalent you're going to have to make your peace with that ahead of time, otherwise you're just going to end up switching back.

KDE is a good choice, and Kubuntu should serve you fine; if you end up going with Kubuntu, I would recommend sticking with it for at least half a year or so before considering switching to something else, as that will give you time to really understand what you like and don't like about how Kubuntu and KDE work.

Steam, wine and dosbox with them, you can effectively play any game worth playing. Beyond that I never needed to know.

I guess the best distro for gamers is https://github.com/ublue-os/bazzite

I'm no gamer but if I were, I'd go with that. Thus could be helpful as well https://fedoraproject.org/atomic-desktops/

The most important thing: Tell us, the community, what your critical application needs are, and get suggestions for applications to use. So many people jump through fifty hoops because they Google search first and the first thing they try turns out to be deprecated, the second thing they try doesn't work on their system, the third thing they try has everything they need minus the most important part, the fourth thing they try turns out to be proprietary and half-broken, and so on.

You will not find good solutions just by searching around, you honestly, truly, need fucking nerds in this community who live this shit daily to help you know what the genuine best available solutions are. Otherwise you will spend weeks pounding your head against the keyboard using the wrong solutions, not because of anything you did wrong but because there are often so many different implementations of the same thing that it's nearly impossible to know which ones are the ones you need for your use case without directly asking some people.

Once you've been using it a few years, you'll be familiar enough with working solutions to keep track of this kind of thing yourself, but trust me, it takes a while. So please do yourself a favor and make a thread asking which applications people suggest for the distribution you've chosen to use and what kind of framework to install them from (repository or flatpak). You will save yourself a lot of trouble.

Also, as for keeping your backed up data from Windows on a USB, I think best practice is to always keep that kind of info backed up on an external drive, no matter the OS you use, or whether you plan on switching, so if anything fails, the drive will always still be there and readable (unless the drive fails, of course).