My solution: https://github.com/vladstudio/Vladstudio-smart-resize-Bash-s...

The biggest problem of scaling down an image is to find right settings for resampling and sharpening. After many expreiments, I found it impossible to achieve good results by simply running convert with a line of arguments. So I came up with this script, which basically does the following:

* configures -interpolate bicubic -filter Lagrange; * resizes source image 80%; * applies -unsharp 0.44x0.44+0.44+0.008; * repeats steps 2 & 3 until target size is reached.