Hmm modular is an interesting thought. How would you go about creating that tunnel in a modular way? BTW, thankyou for the feedback, this is really helpful!
Update Made and textured some modular tracks. Been playing around with layout in UE4, with some atmospheric fog effects. Obviously just skeletal at this point. I still have to make rocks, plants etc.
I'm still getting familiar with different workflows. Masks do sound like a solid idea because of how large the asset is. I'm also curious if decals would be a good fit? Me, I would make this modular. I would paint everything in 3D Coat. And you can be deliberate with where everything needs to go.