in your case a ceiling fan, the patch will always be higher than the existing ceiling, there's only two ways of dealing with it Jim.
One way is when you put the patch in place make sure it is lower then the existing ceiling, you can do this by using a smaller thickness piece of drywall, example, ceiling is 5/8 thick, patch can be 1/2 thick, doing this will allow you to just fill in the 1/2 patch with mud to the existing 5/8 ceiling.
The other way is to keep floating mud around the patch and out woulds until it looks better
( until you don't see the hump as bad where the patch and ceiling meet )!
But in most cases if you don't do it for a living everyday you'll still see the hump where patch and ceiling meet, repairing a patch on a ceiling is tough even for the pro's, my eyes are still train to see flaws in drywall where ever I go Jim, even on newer homes, they stand out to me like a sore thumb.
You can try and hide the repair with a thick texture or even paint swirls, the smaller the swirl the less you will see of the hump but it's not totally 100% hidden but good enough, hope this helps Jim, one thing is for sure, after many years at drywalling I'm glad to be out of it
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