Not a caster, but definitely the laziest and most relaxed build I ever played (and also one of the strongest):
Maya’s “Fluffy Squishy”
It’s a pet build and it has a lot of buttons. But most are toggle buffs and pets you don’t have to resummon often. Gameplay is really easy, no button smashing. Besides that, the build is not that gear dependent. For optimal performance you need the items in the GT, but the build will also do fine with rather basic equipment: Just get as much boni for pets as possible and cover your resistances.
In general, “caster” and “lazy” is somewhat contradictory. Casters have to cast spells to deal damage or defend themselves, so every caster build I know requires a certain degree of active gameplay.
Of course there are rather sturdy builds, that don’t play like a piano, for example PRM Mage Hunters:
PRM endgame build by mad_lee and
Beginner’s and leveling build by Nery
AAR mage hunters or Spellbinders could also fit your bill:
Endgame Spellbinder by ridopenyo
Lightning AAR Mage Hunter by Nery
Chaos AAR Mage Hunter by mad_lee
Anything using AAR is very easy to level. AAR is a very strong skill on its own. It will carry you through campaign and early endgame even with suboptimal gear.
Of course it’s up to you, what and how you like to play. But I’d recommend getting at least one character to level 100 and early endgame. It will make leveling alts easier: You can start farming gear. You also can buy xp potions, that double your xp, if you are revered with Malmoth Resistance. And in Forgotten Gods you can buy a token that unlocks every waypoint in Normal/Veteran and Elite and also gives you skill points from quests on these difficulties. So once you’ve reached level 100 with one char, leveling alts will be much easier and faster.