Haha, it took you longer to do the backdrop than the car! I’ve started this car twice and gave up because it looked terrible. I’ll get it someday. Very nicely done!
It was tough to get the correct shape, and there were definite challenges. But getting a back-drop I was happy with was really challenging. I tried the smoke-sim for the first time, and it took me a couple of weeks to get it looking like it is; I tried modelling some clouds first, and while they looked ok, they weren’t right.
With the ground, the cobble texture was probably a little low, so it was a pain to get some ‘life’ into it. It looked like a texture slapped on a plane for ages. I’m still not hundred percent happy with it now - but I like the cobbles in that texture.
Cycles (4k samples); I used a render farm and as it tried to switch from gpu rendering to cpu rendering near the end; it took longer, likely cost more too. Fortunately I had some free credits as I beta-tested.
Given your intitial post, i will dispense with the minute detail nit picking. Instead, i will suggest that join the car modellers group. We would love to have you. It is something that we are trying to get off the ground: