I have never taken a photo of the moon that i have been satisfied with. I don't know if its my lens, my tripod, just always having shitty atmospheric conditions in DFW, or what. Always pisses me off.
This is the best one i got tonight. Canon 7dmkII with the Canon EF 100-300mm f/4.5-5.6 USM mounted. taken at 300mm f/8 ISO 1600 with a pretty heavy crop.
First the obvious. 300mm is not that great for a decent moon shot when it Is high. I stacked two 2x TC's to get my bucket list moon shot with my 300 several years ago. I had to work very hard to get over the IQ loss in PP. You don't need 1200 but 300 is a tad short.
What was your shutter speed and were you shooting in live view or normal? Did you use mirror lock up if shooting normal? You have some movement which you can see in the stars on the right. They are elongated. Next chance try it again, put it in live view at max magnification and you will be surprised how much vibration there is. Any heavy cropping will show the imperfections more.