I see pretty much the opposite. There will be strong selection for expansionist cultures and species. As long as the universe is mostly unoccupied, expansion

I see pretty much the opposite. There will be strong selection for expansionist cultures and species. As long as the universe is mostly unoccupied, expansion means increasing your share of the total population. Relatively static cultures will simply become irrelevant compared to their expansionist competitors. The experience on Earth recently is in the context of a world where all places that can be occupied (at the current tech level) have been. The selection effect will operate at the biological level as well. Currently on Earth birth rates are declining. Modern civilization is acting like a kind of pesticide. But like any pesticide, it's going to select for resistance. This will likely mean more people will have an increased desire to have children. Selecting for this may take a while, but on the time scales involved in this thread it would happen very quickly. In the long term, all societies in which selection is allowed to operate will push up against limits and be in states of Malthusian equilibrium. To counter this would require an unreasonable uniformity of control across time and space. All ET species, at all points in their history once they can undertake colonization, will have to exert rigid control on their societies and biology. Any defections from this uniformity will explode exponentially and overwhelm those who are not controlled. Metallicity is not a good argument, since there are zones, even in our own galaxy, with higher metallicity. Closer in to the center, for example. https://www.nature.com/articles/s41550-023-01977-z https://www.nature.com/articles/s41550-023-01977-z