Welcome to the club, and great see you to introduce yourself with a game-idea!
If I may: don’t fret over your main concern of accidentally duplicating another game. I can’t think of one and – more importantly – you’ll never create an exact copy. Boardgame mechanics/theme are not subject to ownership and the entire industry is based on (re)mixing existing things that work.
The theme sounds great – a dying earth feels like something that would work well with some urgency on the time-line of the game. Instead of your default ‘7 years/rounds’, you can be more creative and think of some event-based timer where your faction needs to have finished their escape vehicle, otherwise a chasm in the earth will kill them.
It could also be action-spaces on some tectonic plates shifting, that uncover and cover some actions you can take during the game (or maybe an action becomes more and more expansive as it drifts away from your base).
Finally, the external event is so dangerous and grand – it would seem weird if there are no moments where you’d want to work yourself towards points to win, but also work together to stop overarching doom. Something like Lowlands, where you try to win with a sheep-farm, while also working together to build a dyke to prevent a flood that will hurt you both: https://boardgamegeek.com/boardgame/242804/low-lands