Gunnery did change, it's one of the things I've enjoyed most with AH3 - IMO it has made it a bit harder, which is as it should be. Sakai said aerial gunnery was akin to shooting butterflies with a rifle, and other aces (Beurling, etc) spoke and wrote at great length about the complexity of aerial gunnery. Very interesting topic IMO, and AH3 has made it a skill that needs to be worked at a bit more, which is great.
Regarding aerial shooting, any type of shooting which involved pressing a trigger requires ironclad fundamentals in order to become a great shot. Of all the fundamentals, grip, stance, sight picture, and so forth, fire control is by far the most important, yet most difficult for many to do correctly at first. Fire control being essentially everything to do with how you press that trigger. I'll link a vid from Rob Letham, one of the best pistol shooters in the world who guest instructed at my school (Sig Sauer Academy) many times, and has taught Delta, NSW, and other SF units all over the world. The TLDR of his video is that pressing the trigger while keeping everything else motionless is THE most important skill. The same applies for aerial gunnery, our adjunct Sig Sauer school taught a class for RCAF pilots about 11 years ago who were issued our handguns, and they used our techniques for training and visualizing the trigger press for shooting the 20mm cannons as well, in fact in the series "Jetstream" you can see one of the instructors quoting the SigARms syllabus absolutely verbatim while teaching another pilot how to strafe the ground practice targets.
HT's solution is a viable one for sure, however you CAN train yourself to keep the trigger movements isolated from the stick movement, it just takes more training and time in order to do so. The video is below, substitute "the gun/pistol" for "the control stick", and the parts regarding remaining motionless apply well to AH3 - obviously we don't get recoil in our stick, so having to maintain a high pressure clamshell grip isn't necessary for pilots, either real or virtual.
https://www.youtube.com/watch?v=li0rGtXh23I