I think you might just be asking too much - you’re trying to draw lines that are super-thin, like about the width of a pixel and you’re wanting them to curve smoothly. I’m not sure it’s really possible, but certainly it will depend on the resampling/interpolation as the image gets shrunk and we only have a little control over that from within OpenGL. @lindeloev’s solution of resizing your image in your graphics program, to the actual size you want in pixels, is the right way to go