You don't really need a cleric in this game as damage or control trump healing in this game.
Moreover with the abundance of food and potions, you should have ample sources of healing. And the help action or throwing food gets up downed party members so 'healing word' got nerfed in that aspect as well.

I did 2 runs in EA.
One with a balanced party (laezel with sword and shield as 'tank', shadowheart as cleric, gale and Warlock PC) and one with 4 damage dealers (laezel with a 2hander, Astarion, Wyll and Ranger PC). The 2nd run was a lot easier and not just attributable to meta knowledge and experience.

So I would build the cleric as a light domain cleric for extra damage. Astarion is a beast as a thief with the correct setup so I'd choose him over one of the casters.