Airbus recommend using auto thrust when manually flying. The children of the magenta line lecture was by a 1990s Boeing pilot to 1990s Boeing crews. Different philosophies from different manufacturers.
Ground speed mini is an Airbus function and is not tied to autothrust. There is nothing preventing a pilot from using manual thrust and targeting the speed bug which may be influenced by GS mini. In other words, you are incorrect when you say “disconnecting auto thrust disables GS mini.”
Where I work (A320) we leave auto thrust on unless it’s not working adequately or we feel like turning it off for practice, most pilots leave it on for most approaches.