Re: The 1920s onwards
If we are going to mention possible sci-fi themes going back into the mists of time - then it should include E.M.Forster. As well as his famous "romantic" novels he also did what could be called fantasy stories.
However he made a significant contribution to sci-fi in 1909 with the short story "The Machine Stops". An uncanny prediction of the social effects of the World Wide Web. Even the BBC "The Book Programme" once misattributed that story to H.G.Wells.