I think astronaut has a higher skill floor than guitarist. I don’t mentally separate astronauts into categories like amateur/average/talented/expert, it’s just assumed that any astronaut is an expert. Leading with describing someone as a guitarist before you mention they’re an astronaut too implies that they’re also an expert guitarist.

So basically it’s the difference between:

• An expert guitarist who’s also an expert astronaut

• An expert astronaut who’s also an amateur guitar player.