ImageMagick. Even has a command line interface! On a serious note, I don’t have any problems with GIMP, 2.10 is supposed to have hiDPI support. Have you tried

ImageMagick. Even has a command line interface! On a serious note, I don’t have any problems with GIMP, 2.10 is supposed to have hiDPI support. Have you tried Krita and MyPaint?