Worst-case scenario: large scene, huge meshes. Tips on baking or computing realtime?

We've got a giant outdoor scene (around 1300 meters by 700 meters), and unfortunately it's made of giant scene-wide meshes (generated from a third-party tool). Are there any tips for finishing even a low quality GI bake or realtime computation, that doesn't involve changing the art? I click the Bake button overnight (or over weekend) and when I come back Unity has crashed/hung, or run out of memory (64GB RAM PC), or both. We're using Unity 2019.1, when just computing Realtime Global Illumination, we are choosing "Default-VeryLowResolution".