Thanks for the video, gave me an idea of how to reproduce the behaviour of the cropping tool in Photoshop. But these are still too many steps...
My steps are:
1. Add rectangular mask with given width and height
2. Upscale mask while holding shift to keep aspect ratio until it hits vertical/horizontal maximum
3. Rasterise the parent layer of the mask
4. Downscale to the desired resolution
5. Create a slice in export persona with desired resolution and export
Pretty much stuff to crop an image with given aspect ratio :/