I wonder if it might have been easier, and more impressive on camera, to use a scale model in various kinds of in-camera VFX shots? I'm thinking of some of the old-style forced perspective shots, like those used in the 1993 Attack of the 50ft Woman with Daryl Hannah. That might have helped with the ruins that "extend to the horizon."
There aren't any shots that require close interaction with the Guardian. The landing party pass in front of it, around the back of it, and jump through it. They might have pulled it off with no need for post-FX work. (Just a few years later, Space: 1999 produced almost all of its VFX without post optical work.) Such shots might have been too time consuming, though, even for the short time the Guardian and the planet appear in the episode.