>I could make them oblong and different colours, but I also can't see how it would help
It makes a huge difference in helping you predict bullets. When the sprite of the bullet tells you in which direction it's going, you don't have to spend those milliseconds looking at them to figure it out yourself and can focus entirely on mapping out a path. Here's an example off of the top of my head (go to 10:45; Itch won't let me link timestamps):
A pattern like that would be much harder with circular bullets. And just having different shades of orange would help with differentiating bullets with different behaviors so you can focus on the more immediate threat.