Hope they work out for you!
Also, I’d say of the three, if you want the one that will “just work” the most, I would go with Kubuntu, mainly because you’ll be able to search for stuff and problem fixes easier since there’s a lot of info online for it, because it’s the most commonly used of the three I believe. The other two are still perfectly fine options though, but I think Kubuntu would be the smoothest experience, as you said you were still a bit new to Linux in your post.
I use Debian now. And for kde neon they add their software via debs as regular packages, but you can add either snap or flatpak to it if you want (or both), can’t remember if they preinstall flatpak or not.
For that part, they’re just saying that other desktop environments are unsupported. Back when I used KDE Neon I had no problems using it as a daily OS, it is technically less stable (in terms of the system changing with updates, not in terms of crashing) since all the KDE software is updated regularly, but the rest of the software is the same as regular Ubuntu LTS.
I’d say the best way to think of KDE Neon is “Ubuntu LTS, with the latest KDE software directly from them, with less guarantee of stability” imo.
Edit: They also say they’re not “quite” a distro, it is by full definition a distro, but their goal is different. The only intent of KDE Neon is to have a stable base with up to date KDE stuff, while a “distro” as they are saying will have a goal of being a fully fledged OS (or similar). Neon can be used as a full distro, but that’s just a side effect rather than the intent.
I’m currently running on Debian Stable, but I was running on Debian Testing until Debian 12 released. RX 6700 XT + i5 11400f. I use debs for most things and Flatpak for everything that I can’t find on repo.
I had no more or less issues on it than any other distro, I’ve never really had anything break but of course YMMV.
As a side note, I’d recommend trying Stable as well to see if it works well enough for you, then moving to testing if it doesn’t. As long as the performance is fine enough for you, it’s really nice having the system not really change since I only have to get a game working once, and it just keeps working exactly the same until the next Debian version with no surprises.
I’ve been taking incremental backups with borgbackup using Vorta as a frontend, was nice and simple to set up and haven’t had any issues with it so far.