It’s actually not bad at all - in my test map the entire scene, with 31k trees plus landscape lightmaps accounts for only around 150mb, then bear in mind that if streamed properly, only a small fraction of that is loaded into memory at one time. This seems perfectly reasonable to me (when/if it works as expected).
Unfortunately AO does not replace good GI, and although manually painting in bounced light/shadows is an interesting concept, it drastically increases the time needed to light and iterate on scenes.