Modifier keys were the first thing I tried (because that's how I learned to do it in GIMP and Inkscape when I was in high school). When that didn't work, right-clicking on the crop box was the next thing I thought to try. It's a total of 2 clicks: right click to open the menu, then move the mouse down 6 entries, then left-click on the checkbox.

But also, it's my understanding that Krita is not a photo editor, but rather a digital painting program; that any ability it has to edit photos is just because digital paintings are stored in the same file formats as digital photographs. Saying that Krita is unsuited for editing hundreds of photographs is analogous to saying that Visual Studio is unsuitable for creating ASCII art. There are other tools that you're supposed to use for those tasks. For editing hundreds of photographs, I understand Adobe's Lightroom or Bridge to be the intended tools, not Photoshop, nor any tools which are highly similar to Photoshop (GIMP, Krita, or otherwise).