Thanks for the details ... interesting ... although perhaps a bit more dire to you 
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1. OK.
2. I can only start to imagine what "spread" means ... never seen something that could fall into that word. Of course, the experiences are from past and might not relate to problems of today.
3. OK. Yes, it was clear as such.
4. OK, end of that path then.
5. Well, depending on what "spread" really means, I can see that this is graphics card related. You know, you have many layers of graphics that they need to deal with and failure to perform can appear very strange.
However, there are many different kind of things that you could try, which all relate to what happens on your desktop:
a) The quickest is to check the driver. Go to Software Sources and there you see Additional Drivers. There you can see which driver is selected ... I just had a recent thing when Kubuntu selected too new a driver.
Change the driver to OpenSource, while it is probably now a Closed driver. That will change the driver vendor and will probably have an effect, if the driver was the problem. You need to reboot to see the results.
With Open drivers you may lose some speed and perhaps performance, but in my case, I cannot see any difference - other than it works now (with OpenGL or DirectX, the story might be different, if those are relevant to your use).
This is also something that the Reinstall is unlikely to fix.
b) Try with another desktop. I'm not a fan of Gnome 3. I did like Unity, but Gnome 3 just sends the ideas back 10 years. So, this could also be a Gnome 3 thingy, with your specific circumstances.
Since you are reinstalling anyway, you might want to try another desktop, most conveniently with another boot system disk. THAT you can easily replace in a laptop, while graphics adapter - not really as you state.
I have moved into KDE, which means Kubuntu 20.04.2. It is different, but much nicer in my view. There the virtual desktops that I mentioned work like a charm.
Obviously, there are many other desktops, like xfce, that could be to your liking. I use that with Ubuntu Studio - it is a bit boring but works.
6. As for viruses&stuff, on both computers - cannot think of anything specific to that end.
As for reinstall, it shouldn't be necessary if the system starts - or even after that if you can hack it through. You could fix the install packages, see if there was anything to fix and then check Update. I'm a meaning a far-fetched scenario where a broken package dependency is preventing required prerequisite packages or otherwise needed packages to install.
I hope you find the cure!