>astronauts assigned to a mission can make money through public speaking
As somebody who knows many current and former astronauts (lived a stone's throw from JSC for 20+ years)... I can assure you that astronauts picking up money for appearances is fairly uncommon. A majority are incredibly low-profile, publicity-wise. If they show up to an event to speak, it's almost always because they personally feel compelled to advocate for research, education, the space program in general, etc.
As somebody who knows many current and former astronauts (lived a stone's throw from JSC for 20+ years)... I can assure you that astronauts picking up money for appearances is fairly uncommon. A majority are incredibly low-profile, publicity-wise. If they show up to an event to speak, it's almost always because they personally feel compelled to advocate for research, education, the space program in general, etc.