A lot of my UI sprites are not power of 2 so Unity might not compress them. However I put them inside a sprite atlas which is 1024x1024. So are they now considered a power of 2 and will Unity compress them? Or should I still convert them manually one by one?
When it comes to compressing, you only care about the atlas size. If the atlas is a power of 2, that should be enough. The individual UI sprites do not need to be compressed separately, so as long as they are part of an atlas, they can have any dimensions.