It can be argued what was seen at the end of ROTS was some sort of test structure. It’s a bit of a stretch that it actually took twenty years to build a station the size of a small moon, and keep it a secret that long.
There is a hint about it in Rebels, when we learn that a super weapon might have been assembled in Geonosis orbit, with all of its population deported… elsewhere (planet Poland of the Outer Rim I guess). I think there will be a dialogue about that in some kind of Imperial situation room (“this project is taking too long Tarkin, I want results !”).