You have a good enough rough opening and may need it depending on how square your opening is .If the opening is square or close to it I would nail a 1X on each side and shim to make it square.Your casing will cover the gap at the top so no need to add.The 1X on each side will still allow 1/4" each side to shim the door jamb.

I’m considering a 54 inch or 56. Inches fan to keep the space cooler I have a lot of natural light coming in with the sun hit and the morning and afternoon.

I drew it up in my program for you. Left shows 14" diameter. Right shows 18" diameter. Depends how big you like "big'. Also, don't worry so much about the height of the fixture because it's the diameter will dictate the height scale of the fixture based on the design you like best. I personally like the scale of the left design better, but that's just me. Do what you like best!

Thank you, @3onthetree, for chiming in!! The pocket door framing has arrived! And the carpenter is going to attach it directly to the joist after taking down the 2x that's currently mounted. Cabinet maker is going to remake the oven and fridge side panels and make the casing (wasn't in the original plan). They will transition to similar panels on the other side (entire pantry will have panels, some serving as doors to cabinets). He is suggesting the casing have a 1" reveal on the pantry wall side to transition to the panels on those walls, but that seems really thick to me. I looked at other elements in my house and no moulding sticks out more than 3/4" from the wall. He did send me these drawings earlier today as options to connect the fridge and oven cabinets - return the crown or keep the recessed ceiling panel. He now wants to return the crown and delete the originally planned recessed panel, but he drew both with the casing to see both scenarios. He also created this cove detail on the vertical casing to keep it from looking too thick at 4.75". I can't tell if his first choice will look good (returning the crown), and I want to go with whatever he recommends, but that's how we ended up with a 36" opening in the first place, which he recommended months ago (and even just a few days ago, a 40" opening with smaller casing, which I now know would have still felt tight, especially in my house, which is not an old home and I don't live in New York City!).