That's where I seem to be missing something. I don't see a "save default" option with the popups on the keystone tool, but it's there with other tools like base characteristics, exposure, noise, sharpening, etc.
Ah, I see what you mean. But I've not had to look at that option for years, since I set it in my default Style which automatically is selected when I import new files.
I suggest you create a preset under the Keystone correction panel. For example make one called Keystone 100pct. You can make a few others if you find that you use other amounts frequently as well. This is not necessary, but may help later.
Now, with that parameter preset e.g. at 100% correction, also set all other settings you would like to always use for a given camera or shooting scenario (such as default noise reduction, tone curve camera profile, you name it), create a user style that includes all those settings.
Then apply that style on import (see first attached example).
Or apply a preset upon import (second example).
By naming the preset and creating the style, you are very flexible in changing settings from the Import dialog panel, or from the relevant tool panel if you are in the middle of postprocessing a series of images.
Cheers,
Bart