If anyone also has a way for QGIS to not display the black areas around the ‘real’ image when added as a raster layer I’d also be very grateful for help with that!
But if I calibrate the image as one big one, it’s severely different from doing them separately when subset, I think the only real option is to calibrate it as one and then subset, rather than the reverse.
It shouldn’t matter for calibration if it’s done before or after subset. I think what you need to do is make sure your visualisation is equvilant for all images. Go to the colour manipulation and assign the same min max to all images.