I understand license changes mean this likely won't be an out of the box feature anytime soon. But if I am to purchase a licence, will there be any issues with running FA6 Pro on XF2.2, or would an addon be required for this to work?
Font Awesome 6 is based on CSS Styles, so I would say it doesn't matter whatever version of Xenforo or any other script you would use as ypu need to manually link FA6 in your themes.
I would recommend getting themes from ThemeHouse though as they know what they're doing and FA is fully supported.