A 18th level Arcane Trickster will do 4 dice of damage with cantrips (even if he's a "1/3 spellcaster")?
Yes. Cantrips scale by character level. So a high elf theif will still do 4 dice.
Though you can't sneak attack with it, so it's not really worth while. Better to take minor illusion to give yourself something to hide behind.
Yeah, to be honest, if you get a damaging Cantrip, Firebolt might be your best bet, since it sets things on Fire. And yeah, Minor Illusion is the other Versatility grab you'll probably want.
But hey! Lots of other cantrips out there! Don't let us choose them for you ^_^ Let your character do that 