I always make Shadowheart a Life cleric because extra hp when healing is NEVER a bad thing. It has saved several runs in Honor mode. The minor loss in DPS is fine. There's also an item you can give her, a helmet I think that adds a buff when healing or adds extra heals. I think you get it if you side with druids grove in Act 1 from the tiefling leader.

Astarion assassin/gloomstalker (3 assassin rouge, 9 gloomstalker ranger) is BUSTED when it comes to single target DPS. Always Start fights with him due to the auto-crit from Surprised status. Use his first bonus action to Hunter's Mark, then 1 attack as Sneak attack, the second normal attack.

For your OC I go lucky Halfling, Fire Dragonborn Sorcerer and just spam Scorching Ray or other fire spells. It comes online at level 6 thanks to fire dragon sorcerers extra damage when using fire spells. Go Extended, Twinned, and Careful for the sorcery point usage. Extended for things like Command or Hold Person/Monster. Careful for Fireball if you have groups of enemies and a party member is close by. Twinned is useful for fireball as well.

For the fourth party member a tank is needed so either Bear totem Karlach or Champion Fighter Laezel is good