[wim]

If your main concern is security, then running Windows (whichever version) is a bad choice
Linux + RawTherapee or Darktable + gimp is a great choice for image post-processing.
Secure, free and extremely customizable.
And if you're super paranoid, then you should consider QubesOS (https://www.qubes-os.org/)

I know you winked, but that is just plainly not true anymore, provided you use a good security suite, like I suggested.

FYI, the worst security experiences I have ever had in my 35 years as an IT-professional, were with Unix/Linux, the last one in 2010 . Whatever system / OS you use these days, you will still need some form of protection against external threats, although that oen in 2010 on CentOS was unavoidable due to a tiny, exploitable hole in cPanel, and an infected server at the registrar I since left behnd ....

As to image processing: the best tools, IMO, are available only on Windows, and partially on Mac. If you insist on using free tools, Windows or Linux are probably best, but you won't necessarily get ease of use or ease of workflow, and neither will you be using the best tools available .

Kind regards, Wim