Thanks for the comment. I have had exposure to similar claims, but wasn't seeing anyone using AMD GPUs for AI unless they were somehow incentivized by AMD, which made me suspicious.
In principle, more competition in the AI hardware market would be amazing, and Nvidia GPUs do feel overpriced, but I personally don't want to deal with the struggles of early adoption.
The mirrorlist is a configuration file listing servers that updates can get pulled in from.
When a package update is installed that contains a configuration file, it will not overwrite the old file but be installed with a pacnew extension so that you can merge the files (like you did). It will keep complaining at you until you remove the pacnew file, which is fine to do after you have merged successfully.
The graphical issues are probably due to something else that happened during the update.