Layered massive Navmesh

Hello,
I am currently working on an “open world VR abstract game”.

And I’ve got some problems using the navmesh system. Because the world has multiple floors / layers, and those are really far apart, we’re talking like 300000.0 cm apart… So whenever I put the Nav Mesh Bounds Volume on my floors, only the one at the bottom is used (I can see the floor lighting green, and can navigate on it using VR controllers) but those above don’t work.

So my question is, do I have to link the navmesh together somehow ? Using Nav Link Proxy (I tried but I might be using it wrong) ? No wrong answers, any ideas are welcome.

Thank you

edit: my floors are detected if I increase the “cell size”, but by doing so I’m loosing in precision and the mesh is clunky.

PS : I cannot use one GIANT Nav Mesh Bounds Volume as every floor don’t have the same dimensions aren’t properly lined up.