Did you look at the bulk import scripts? They might have a smaller memory footprint.
But it is true that the import scripts keep in memory a map of old user, topic, and post ID to the new one so that’s a bunch of ram, especially if you want multiple copies.
You understand that after you run the initial import you’ll run it again to import just the new data and it’ll run much faster, right? So after you wait a month to do the initial import the final one won’t take as long.