Big Sur is prehistoric at this point. FFXIV's Mac wrapper likely got updated with this patch as part of the changes to better support Apple silicon and as such it's probably targeting a newer OS release than what you're running.
If you're running a 2014 iMac and unable to update, about the only option you have is to try xivonmac and see if that performs better for you. Otherwise be sure to check your settings and make sure things like MSAO are turned off in the game settings, if in doubt toggle to laptop high and see if that helps.