a courner of my original SF universe where I can write plausibly almost-fantasy stories while still having spaceships and everything

As Star Trek and other shows have so thoroughly learnt, this is what "the away mission" trope is made for! :D Beam (or stargate, or land on, or be given a diplomatic assignment on) Planet Fantasylookalike, and voila - all your favourite fantasy story tropes get to come out to play, without sacrificing all your favourite SF tropes!