Hello.
Some update on progress.
First to answer a previous question. When you place the trees using the paint object of 3ds Max, you need to untick "align to normal" in the "Align option". It will paint the tree without them following the slope. ;-)
On my side I finished cleaning the mains meshes of the ground and road.
I've been a bit further than expected to be sure there is no holes.
I unified all the mesh under one and fixed it until I have a STL check without errors, then re separated it into its different meshes. It was no small task

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I also fixed UV on ground object (visual road, grass terrain etc...) where I could.
I also organised the max file with layers.
I made the small cabin next to the finish line (it needs a little bit more work but the main stuff is there).
I also made the concrete block along the pit lane. They don't have the right size (I stupidely made each block a littlle bit longer than they are in reality) but I will fix that. I have a 4 textures variance along them (under one texture of course).
Today I'm working on making the physical wall around the track with simple mesh to stop using the visual barriers as colliders (and save a lot of cpu ressources...)