Visibility queries on merged bounding boxes...is it possible?

Question

I have a very large scene to render (approx > 10-20M polygons) , using three.js , with an acceptable framerate.
So I am implementing some basic occlusion culling using a WebGL2 renderer . and here's my problem :
I want to squeeze every performances I can from three.js. In this scene , there are hundreds of thousands of different individual meshes .
Every mesh has to compute it's own bounding box... that means hundreds of thousands of B-Boxes ...
And I don't think that issuing hundreds of thousands visibility queries would be very fast.
So is it possible to merge all these bounding boxes , send only one visibility query draw call , and somehow retrieve every bounding box and it's own visibility query result?