Regarding the stuff you want to do: it doesn’t matter what distro you pick, they can all run the same stuff anyways.

I would recommend something Fedora-based, because that’s most people recommend. It’s very sane and user friendly. Bazzite would be the best pick imo, but the classic Fedora KDE variant is also good for most people.

All other of your questions, like gaming stuff, are (or will) be answered anyways by a dozen other people here.

But I can give you some perspective regarding CAD and video editing.

CAD on Linux sucks. Most is made as Windows only, and many people have tried running it through Wine (Bottles) or other janky methods. Don’t even try it.
FreeCAD is also not the best choice in my personal opinion, especially if you come from other CAD software. But you can try it of course, maybe you’ll like it.

The only “proper” recommendation, and what I also use personally, is Onshape. It’s browser based, intuitive to use, has a good UX and many features. Downside: it’s proprietary and browser based, including all your files. If the company behind it decides to piss in your face, there is nothing that will stop them.

Video editing is a lot better.
As you already mentioned, there’s Davinci Resolve, which is available for Linux too.
I recommend you, especially if you choose Bazzite or any other distro where it isn’t available officially, to check out Distrobox and install it from there, so it won’t mess up the rest of your host OS.
But maybe try Kdenlive first. It’s a one click install and works great too, while being FOSS on top.

I’ve had something similar a while ago. Trying a different distro or doing stuff on the software side didn’t help.

What fixed the issue was getting a new hard drive because the old one was breaking down.

But maybe try other approaches first

None at all tbh, at least if you use the PC alone and don’t share a lot of stuff with Windows devices. If you do, then maybe scan .exe or other files (e-mail attatchments, etc.) with ClamAV or similar to prevent spreading stuff.

You usually don’t need AV software because you install stuff differently than on Windows. You don’t hunt .exe-files from random internet sites, thats irresponsible even for Windows.

You install your apps directly from your software center (a frontend for Flatpaks and repo software), where they usually are pretty safe.

Also, sandboxing is a thing. The prefered way for most people (and often default) is via Flatpak, where apps are restricted on what they can access and do. You can lock them down even further if you want.

There are more ways of sandboxing, but those are not so relevant here right now.

Also:

• If you run a script, check it first. I have zero clue in regards of coding, but even I can usually guess what each line is supposed to do.
• Don’t add 3rd party repos if you can, use containers instead
• Go for the easiest route, guides for “Linux” aren’t noob friendly. In your case, search for “Mint” instead, most stuff is pretty easy there.

Of course. That’s why it’s called KDE Plasma and not KDE Liquid or KDE Gaseous

100% AMD, for sure. AMD won’t make much problems and works ootb.

Nvidia on the other hand… if you already have a Nvidia GPU, then the proprietary drivers work pretty well, but even those won’t work flawlessly and still cause problems for many people.
And the FOSS drivers are still in the early stages and won’t cut it. So why spend lots of money for a piece of hardware that won’t give you the performance you paid for?

Also, Nvidia clearly doesn’t care about PCs or its’ users, so why support such a shitty company with your money?