The only character who can match her damage output in my party is Astarion with Sneak Attack
Dude come on.
Astarion as a single-class Rogue (any Rogue subclass) has
terrible damage output! Appalling. If you're measuring against the worst DPR of a damage-oriented character in the game - a single-class Rogue, sure THEN a Cleric looks good! But Fighters, Barbarians, Monks, Warlocks, Sword Bards, Beast Rangers, and a vast array of others all also wildly outperform Astarion and his sad little Sneak Attacks. The man is getting by on good looks and attitude!
Anyone with multiple attacks, for example can benefit from Haste/Potion of Speed (which are common and easy to get hold of), and become an absolutely terrifying machine (even without it, stuff like GWM will often allow them to easily outperform a Rogue). Even without that, what's really sad is, because Rogue SA is 1/turn, Rogues don't even have good burst damage compared to actually burst-y class and can't really benefit from Haste.
You can engineer Astarion into a pretty good killing machine (one thematic way is Assassin/Gloom Stalker/Fighter mentioned above, but it does rely on you getting Surprise, and thus not talking to people you attack), especially if a
certain incident goes the dark side way (+1d10 damage per attack is nothing to sniff at!), but if he's your top damage dealer as a single-class Rogue well... < frantic blessing motions >
So your problem here is your point of comparison, imho.
Clerics are absolutely great at mowing down undead or anything Radiant-vulnerable (including at least one major boss who is otherwise hard to deal with), with the spells you listed, I will give them that - Heavy armour ones (so not Trickery) also have the AC to stand in dangerous places. But beyond that? They're just a like a bad Druid. A Druid can easily get stuff like Haste and Ice Storm, which a Cleric can only wish they had.
This is a BG3-specific thing note - in BG3 Ice Storm and Haste (for example) are both way buffed over either 5E, or other spells. I often play Clerics in 5E, and they're a good class - they're just really outclassed by Druids in BG3, even before shapeshifting gets involved.
You know who has been a great disappointment to me in combat?
Yeah that is really sad.
I dunno why they felt the need to give her and Minsc 5E statlines - they should have made them both impossible to respec out of Druid and Ranger respectively, but with unreasonably high stats to help make up for it.
I was literally about to post how Shadowheart's story is great, but how she is in combat just doesn't work. She's not good on the front line or ranged, and is meh at buffing. I've been playing an all-custom group, and battles are going significantly better; mechanically the main difference is that I have a druid in Shadowheart's slot.
I swear half the problems people report with combat in this game boil down to having single-class, unrespec'd Shadowheart and Astarion in their party because they're cuties. It's kind of a mean trick that Larian pulled imho. Any respec to Shadowheart, literally to any other domain is a noticeable upgrade (why make Trickery so bad if you're going to give it to a main character!). And Astarion as Rogue wouldn't be that bad but all the ways they've improved things for melees - OP version of Haste and tons of magic items that buff melee - don't really help Rogues, because they only have a single SA and a single attack. I haven't even seen an item which say, increases SA damage. If you had an item which added say, 3d6 to SA, and another item which gave Swashbuckler-style SA conditionality, suddenly Rogue would be decent in Act 2, and if you had an item giving say +5d6 to SA... well he still wouldn't be great for Act 3, not as a single class Rogue, but he'd be okay - or how about just letting him SA twice (bonus action attack to land the second)? An item that did that would be enough to make a single-class Rogue good.
They also buff casters through the sky with OP Haste and loads of items giving +1 spell save DC and to hit with spells, and various bonus damage stuff, but poor old Rogues? Nope.
