I made a panorama(of the GoG skyblock) for a main menu, but some of the planets are cut off and the rainbow doesn't line up. Is there a way to fix this? Or is this something that isn't possible to fix?

try changing the FOV to 90. it worked for me :)

Unfortunately for this one in particular I don't think it can be fixed. The render is a bit finnicky. Sorry for that.

You need to find a custom FOV so that the images stitch properly. Use this video to learn how to test FOV based off your world