Q.Gold Dust Croton Is Dying.
I bought a home with two gold dust (many years established) crotons in front in Florida (Zone 9b). About three weeks ago, the leaves of one of the stalks of the plant started to wither and drop. (These plants had received no attention from the older person who owned the house before and when I got there a couple years ago, I trimmed them to promote filling out and new growth. I water them during dry periods. Once the first stalk had dropped it leaves the problem seemed to move to the other stalks. I checked for the obvious — too much water, not enough water, bugs. The soil is sandy and well drained. The plants have been here for many years. There are no bugs and the other plant (just a few feet away is fine — though there is some sooty mold left over from a bug problem awhile ago.) This plant appears to be dying and it was all very rapid — weeks from first signs to now.

Certified GKH Gardening Expert
Unfortunately, the pruning probably took a large toll on the plant. They do not like any change, whatsoever.
There isn't much that you can do to help them to recover. They will recover on their own, if they are to recover.
Since you are in an area that they can survive, naturally, the best course of action for growing these is a hands off approach.
You can try plant vitamins such as B1 (it isn't in the same concentration as it would be for a human, so human vitamins will be a no-go.) Sometimes they can aid in recovery, but this is not always the case.
This article will help you with their care:
https://www.gardeningknowhow.com/houseplants/croton/growing-croton-outdoors.htm