What of offsets (or spacers) would be needed for a 2018 Coupe for 20" x 10" fronts and 20" x 12" rears? I tried searching and couldn't find it.

With a 20x10 front and 20x12 rear the tires you'd be looking at would require 275/30/20 front tires and 305/30/20 rears. The 12" rear will limit your tire options quite a bit and youd have more to choose from if you were to go with an 11-11.5" setup in the rear. Here is an example of what the vehicle fitment would look like with this tire setup. As for the offset, send me a pm and let me know how aggressive you were trying to go. Your configuration of wheel will also tell you what offsets are possible (1pc/2pc/3pc/3pc w. Floating Spokes) I can confirm with our engineering team to see if what your looking for is within range of being possible.

What is the offset they have on that setup? I think that would look fine, wouldn't want to get too far outside the wheel well.