Re: The 1920s onwards
+1 for Alistair Reynolds. Great space operas.
Another favourite is C.J.Cherryh. The Rimrunner universe is a firm favourite of mine. I have an Aldiss first edition of Heliconia Summer sitting around somewhere. The version of 'Spring I have is a reprint, unfortunately. But his works were imaginative and engaging.
I think that Science fiction is still going strong, it is just harder to find those that don't feel re-cycled because, well, we have a greater wealth of what we consider science fiction to choose from. But a good story is a good story, the background is incidental.
A few recomendations, that if you haven't read, I strongly recommened:
A Canticle for Leibowitz, Walter M. Miller.
The Postman, David Brin.
The Eyre Affair, Jasper Fforde.