Sorry, this is not perfect, ... yet
The idea of sizing up the buttons is a great idea, and it can result in an excellent solution if you follow more cognitive rules of thumbs.

Our human brain is a bit more complicated, so upsizing a button is unfortunately not sufficient for it to be perceived as of greater importance.

A button could be perceived as an important button if it:
* sticks out from other buttons. (in placement, coloring, size, etc.)
* looks like a button (3D effect signalling that you should press it). Unfortunately your solution misses this as the standard 3D hint is designed for buttons of smaller size. Such a big button needs a greater hint so the brain still instantly recognizes it as something that obviously should be pressed. It should also keep the same form as other buttons, i.e. rectangular.
* doesn't resemble other elements too much. (Use a different color scheme, bolder font, useful icons etc.)
* really sticks out from the background. It should certainly not have the same colour as the background.

When I looked at the screenshots, I didn't perceive your buttons as buttons, thus the usability was lowered. :-( They more looke like some decoration.

Keep up the good work!

Your points are good, but implementing most of them would require more wide-sweeping changes to KDE's GUI style as all of the things you mention would have to be implemented consistently across all KDE applications. Not just one dialog.

The screenshots provided here are merely krita mockups based on my preferred widget style, iconset and color scheme settings (yes, I like greenish and brownish colours a lot).
If you were to, for example want to have a more pronounced difference between the button and background colours, you could easily set it according to your preferences in the KDE Control Center as well as choose a widget style with more of a 3D effect to it. Also, the icons I use are fairly non-standard (noia-warm) but they are the standard icons displayed in such buttons across KDE.

I do agree with all of your points except for one: I see absolutely no reason usability-wise why GUIs should be restricted to rectangular buttons only. (for one we already have square icons in all toolbars)
As you described, a button can be recognised in many ways (different colour, pronounced border, icons, etc) and thanks to KDEs extremely unified look and feel, recognising buttons of non-standard proportions is very easy, as all buttons share all the other characteristics even though the proportions might differ.

Anyway, thanks for your comments, which would be an excellent primer for a more general talk/suggestion about differentiating important GUI elements.
Here my goal was to simply offer an idea to increase the usability of one KDE dialog by bringing the relative sizes of the various GUI elements more in sync with their relative importance.

This button is *not* far more important than the others, so I don't think it should stick out too much from the rest.

The main problem here, usability wise, is that right now the button is after a VERY long white space and really off-center, so you easily miss it.

Putting it as the first element is enough to make everyone able to find it. Big size is a plus, as is the written description of what's the button for.

and move the big button to the top