Just FYI, ONK (onenerdykid) is the Atomic ski boot product manager, and is the best qualified person in the world to answer questions on Atomic ski boots. He is also an ex-bootfitter and very un-biased.
I can relate my experience with choosing a shell size for a new Redster Club Sport boot. Trying them on is really important. In the smaller size, I found my toes were crunched both lengthwise and width wise. I could only be in the boot for 5 minutes max. (this is without any work done on the boot). The next size up was very comfortable and my toes never touched the end of the boot. I went with the larger size as it was also snug everywhere else. I have a very responsive and comfortable pair of ski boots that I enjoy being in all day. I am also sure with a lot of toe box work I could have made the smaller size work too. I bet my toes would not be "comfortable" for all day use, though. I am not sure if the fit of the smaller boot would have made it ski any better, I am completely locked down and snug in the larger boot, so in my mind there was no need to go smaller. I probably could have set the buckles looser in the smaller size for the same degree of snugness.
In the past, when there were very few width choices, everyone went as small as possible to get a snug fit. You have a wide foot, so you need more room in the boot. MY son has a very wide foot and skis one size too large to get enough width. From what you describe, you are probably in the right shell, but if you are feeling movement, then that indicates going down a size.
I also put a pair of zipfits on and tried to get into the smaller shell size. I could not physically get my foot into the boot. The zipfit was too high volume and would have needed work to make useable. Others have described the process, and you probably have gone through it.
It may not be where you want to go, but the new Professional boot series from Atomic in 2022 might work well for you also. Since you have a zipfit, it might not be the best option. I suspect compared to your old boot, the new Atomic lineup has something that will be a big improvement and considering all your options with a good bootfitter is your best approach. Check their 2022 lineup if you haven't already (Rester Club Sport, STI, Team, and CS Professional)