How about food importance = min importance = 40% beginning and 10-20% end. I prefer min having a slightly more importance.
That is accomplished easily enough by having highly productive Ultra Rich+Very Rich mineral worlds and making terraforming very difficult.
Because if Ultra/Very Rich worlds are highly productive,
and mineral productivity keeps up with mineral need,
then only 10-20% of the pop would be needed as soon as you colonized some of those worlds.
And unless food is going to have some use other than keeping people alive, it should only increase in productivity may be 5* at most.