Hey @Gavi101
(i) the Glow is the artificial lighting (+density) to create god-rays, which might be a bit strong sometimes. (Lighting might be changed a bit soon). You can change the lighting properties in the Create Instance ^^
(ii) The pixelization when flying far away is changed with the Resolutions by the distance, there are 3 levels currently (near, mid and far resolution). You find these settings in the Render Instance. Slightly raise the resolution at the desired point in space to make the nebula sharp and clean especially on the distance
(less rays are sent when the nebula is smaller on the screen, so the resolution can be increased)
(iii) I think you can try the settings of the previous version with the 4.20 download? Maybe Schlabb could compile and upload the previous version to UE4.21 and upload it on a drive, but this would also take up some time, so only if you really want to dig into that.
Hope that helps 
Lee