After c5. Have added a level of nothing but planters and mealwood plants. (Planters instead of farm tiles cuz it's not permanent.)
When that's finished, we're basically good for another 30-40 cycles. We have o2, h2o, food, and morale. Everything else is just an enhancement.
So.
1) Keep digging. We want the whole temperate biome cored, really.
2) Decide what to do with h2o for the mid term.
3) Decide about mid-term food.
4) Explore explore explore.