I think you did a fine job here! This is unique FG for a comet and it looks great! There wasn’t much detail in the tail on this one and it was never very bright, with poor timing with moonlight. It was barely visible to the naked eye in my only-moderately light-polluted area and I can’t imaging what it must have been like in the Los Angeles area!
As to the crop, the vignette at the top helps pull the eye down to the comet, but is it better with most of the vignette cropped out? Honestly, it’s 50-50 for me. But then, on staring for a while, maybe I do like it more with some crop from the top – but not too much. Just leave a trace of the vignette. Maybe this is one for the Landscape people…
I had another thought – the vignette is mostly from the lens, but the sky is naturally darker farther above the horizon, and often dramatically so when the contrast of a digital night sky image is increased. You might use a feathered selection to darken the sky a bit farther down. If that darkens the top corners too much it could be possible to selectively lighten them with another feathered mask.
