Great! I believe that your experiment "to see the effects of changing a Minetest world into something otherworldly" can be considered successful. It also has educational values. Simple code is easy to understand and everyone can learn what to change, so that you can jump on the moon and admire the huge Saturn so close. That makes great feelings :)
But if you wanted to add more realism, you could add more type of ice: in addition to water ice (H2O), which is the hardest ice in space away from hot star (and on Saturn's satellites, it acts like a base-rock or ground), It would be nice to be able to find some areas of surface covered by carbon dioxide (CO2) ice blocks (which should lie on blocks of water ice like snow on the ground on Earth), and some deposits of nitrogen (N2) and methane (CH4) ice blocks (which should form heaps with a soft consistency reminiscent of small hills, or maybe even small mountains, resting on a ground of water ice and which can move or slide on this ice-ground, at least in some places). Water ice block should be the hardest and should need a pick, but nitrogen and methane ice could be dug also with a shovel I guess. These ice differ not only in hardness but also in color. Carbon dioxide ice should be more white, and methane more blue if I am correct.
These carbon dioxide, nitrogen and methane ices should not be too high or too wide, most of them should be submerged in water ice. (It is not a Pluto). So most of them should be embedded into water ice (which is kind of ground). And they should be there to be able to sublime and create an atmosphere when the temperature is right, as in the case of Titan. Therefore, they should not be omitted. :D
On the surface it should also be possible to find reddish areas containing a shallow layer of simple organic compounds that arise from the reaction of methane, nitrogen and carbon dioxide with the energy of cosmic rays and solar wind (that is, after bombing the ice by protons and electrons from Sun and gamma rays from deep space).
The surface should consist mainly of ice. Ice blocks (mainly water ice) should form the upper layer going down to, lets say... -100 or -300 deep. Below the ice blocks level it should be liquid water, going down to... lets say... some about -500 or -1000? And the next layer, below liquid water, could contain some kind of silicate rocks (stone blocks and stone_with_ores, caverns and so on). We can also consider a different type of geology of such objects and forget about the layer of liquid water. Then, to be more playable as a game, the top layer could be made of ice blocks from the surface to ... lets say -20. Then, below those ices, the stones and ores could be found.
Depending on the geological model, cryogenic volcanoes may also be included, rather as rare things, which should throw liquid water and it should go up really high (let say up to +300) and should freez in flight and fall down to the surface as ice blocks or particles.
Well, my imagination took me far, far away, lol. I guess no one would write such mapgen ;)
Anyway, thanks for your experiment! Great job! It is a lot of fun to run this and just walk there (with those gravel all around - it also looks great) :D