Love the game. Seems now all that's needed is some danger elements.
Ideas to solve that and give the game a "gameplay loop":
A. Something chasing you around the map that slowly gets faster and faster, and the only way to stop it is to develop some kind of gadget that takes most of the materials. If you get caught, it's either game over, or just a loss of some materials. Maybe one of the girls gets hurt and is taken out of the sex and drill minigames for a couple ingame days.
B. Some kind of timer, like "X days left of oxygen/rations" or "X days until the MASSIVE SOLAR FLARE" and you have to do various goals to postpone the game over and eventually stop the countdown.
C. When the game starts, one of the girls is secretly marked as the "infected" character, maybe they're carrying Terminal Space AIDS or are unknowingly host to an alien parasite, and you have X days to identify and cure it, since it's assumed everyone on your ship thing has it now.
C (2). Maybe the girl picked is actually EVIL and wants all your stuff, so you have to identify who it is and complete a series of sexual goals with her to postpone and eventually stop her from enacting a plan. Fucking her onto your side, basically. Goals like "stay in Y sex act for X seconds without losing or locking" and "get a total of X stars across all poses."
D. As you progress through the game, more and more natural events happen, like meteor showers, earthquakes, volcanic eruptions, etc., and you have to manage and upgrade your driller's armor faster than the events can damage it, in the hopes of outpacing it long enough that you make it past the point when it all stops and everything goes peaceful.
E. Rather than a vague "run out of supplies in X days", have an actual food (+water?) mechanic, where you have to keep up on food stores the whole game, finding more ways to get, produce and cook food, and each new girl you get in your group means you can do all this a bit faster, but you also lose more supplies quicker as they use it up.
F. More maps than just this one, each one getting harder than the last. You get a small selection of girls on the first island, and maybe one or two per island after that.