Been asked to possibly make this cake for a 30th birthday. My thoughts are two just stack 6-10" rounds to serve 40. Any thoughts?

Also, any thoughts on what they used for the ice?

It looks like that is just a one tier? So maybe you could do a 10 inch round that is taller, say 5 or 6 inches tall. That should provide 40 thin slices?

for the ice cubes, do you have a mini ice cube tray? I would imagine you could make a sugar syrup mix and as long as you dont cook it too long it shouldn't go dark? but I'm not sure.

here's what I found - heat half cup of sugar with half cup clear corn syrup just to the soft crack stage (250 with a candy thermometer) then pour it into the tray and leave it to set for a couple hours. Not sure if the tray would melt though ... I suppose it depends on the sturdiness of the plastic?

Also a double tier 8 inch might be cool (separated by a cardboard circle so it's cut as two 8 inch cakes)

If your 10" rounds are 2" high, I would think three layers would give you plenty of servings.  Ice looks like isomalt.  You could also use gelatin, but not sure it would come out so clear.