That background is way too dark to extract with, especially if you are wanting to put the images into a lighter background. The problem is the darkness of the background mixes with the edge pixels. Even if the transparency mask of good, there will always be a dark fringe unless you really contract the mask edges.
In general, if you extract from a dark background it will look best in a dark background. If you extract from a light background, it will look best if you put into a lighter background. I thin a lighter gray givers more background flexibility that white or dark gray. Something that samples around 75% on the brightness if you take a color sample in Photoshop.
So if you are setting up the screen to do extraction then that is what I would do. If you want the images on a dark background because you use them without cutting out too then I understand why you do it. However, you will always be fighting this issue I think. There is no magically way to make extractions from a dark background that is nearly black, look good against a light color.
That’s probably not the answer you want. However, I think that just is the way it is unfortunately.