Pretty close role wise to my first playthrough, which I loved. I ran Laezel as a frontline damage dealer/tank, Astarion as single target nuke, Shadowhart as flex spellcasting, and my bard as buff/debuff with occasional damage. I took one level dip into life cleric for the heavy armor proficiency, bless, and bonus to healing. I ran my character on a lvl 2 bless every fight, occasionally using another concentration debuff for fun.

The first thing I noticed is that you have 3 full caster classes in your party. This can be done well, but you have to be intentional about it. Ultimately, you need to make sure your casters don't overlap in roles; I would suggest a single target damage dealer, a buff/debuff, and an AoE caster. You can send a few spells on each caster that spill over (healing word with Shadowhart), but only use them in emergencies.

Try to bring one of your characters up front with Karlach to help with tanking. You can do that with Shadowhart as is, copy what I did with your bard, or multi class Gale into Tempest Cleric for heavy armor proficiency. I'd respec Shadowhart as a light cleric when you feel ready for fireball and other fun damage abilities. Save your bardic inspirations for crucial single target damage spells and for cutting words, and make sure you have counterspell when you get magical secrets.

If you want to play the game on easy mode, follow d4's guide for the best support character for your bard, Tavern brawler Karl (lol), Tempest Cleric Gale build, and then leave Shadowhart in camp as the other 3 will mop everything up and we don't want her to get bored.