Because of the non-square pixels of the RG280V the image would need to be scaled up to 360x320 to look perfectly square. To increase the sharpness it would be best to do an integer 6x upscale to 768x768 and then do a bilinear downscale. I noticed a big increase in sharpness while doing something similar for the GPIcase's screen.
Here are two images, one bilinear upscaled from 128x128 to 320x320:
Tthe other is integer-upscaled to 768x768 and then bilinearly downscaled to 320x320.
It looks pretty sharp already. On the 2.8 inch screen of the RG280V it will look even sharper because of the high DPI. You could do this approach for other platforms as well.