"At the risk of having missed something on this thread, is there a
reason you couldn't install a GFCI breaker in the main panel - for the
circuit feeding that fixture?"

One 15A feeder for the whole upper floor of the house. Think 1950. I
don't want a GFCI for a whole floor in the house and don't have the
spare room in my electrical panel for something that big. I did put
one in for the kitchen so I do know about how much space they take up.
Besides that, you don't need GFCI for a house built in 1950. Code
doesn't require a retrofit. We are getting way off topic here.