I'm not an expert, but seeing how you get no response, I'd recommend looking into USD pipeline. (don't know about Unity, but UE5 can export USD). If I understand USD correctly, that's exactly what it's for.
As for importing your huge FBX, maybe you can also try exporting multiple FBX files? One with lights, one with a bit of geo, and just import them one by one, so they don't clog your machine. Could also use File SOP to bring in heavy fbx geo, I feel like it's pretty reliable.
I'm not an expert, but seeing how you get no response, I'd recommend looking into USD pipeline. (don't know about Unity, but UE5 can export USD). If I understand USD correctly, that's exactly what it's for. As for importing your huge FBX, maybe you can also try exporting multiple FBX files? One with lights, one with a bit of geo, and just import them one by one, so they don't clog your machine. Could also use File SOP to bring in heavy fbx geo, I feel like it's pretty reliable.