Not going to spoil anything but progressing in act 2, depending on your choices you might have something closer to what you are describing.

Darkness needs a party that can work with it. The Eversight Ring and Warlock's Devil Sight eldritch invocation both will preserve Advantage and let you shoot in/out/through it. There's also a helm very late in the game. As the other commenter was alluding to, there's a key component here, and it sounds like you haven't picked it up yet.

I tried this build in my first playthrough and ultimately I wasn't thrilled with its performance. Darkness is smaller than the big surface spells (Hunger of Hadar, Spike Growth), and a big part of the reason why those spells are so effective is their size. Put most (or all) of your party in Darkness and it's quite cramped. And not very mobile. I think if I were to try it again, maybe I'd put it in more of a tanky/control build. Combine Darkness with a Ranger's Entangling, maybe.

If you can handle the deviation, getting Shadowheart out of the Trickery domain would be a big boost regardless. I don't think War is way off. There's a mod adding the Death domain, if that sounds any better (though I haven't tried it).

Respec her into tempest domain. Then channel your inner Palpatine as you zap people after sending a bit of rain their way.

I'm going to try a cleric/rogue for my next playthrough. You don't need darkness really. A rogue would fit her shady nature. Sneak attack works with invoke duplicity. Dual wield would allow the use of off-hand after invoke duplicity. Blessing of the trickster allows for an ally to sneak with her. Likewise pass without trace. Mirror image helps for fighting in melee. Blindness or hold person are great with sneak attack. Silence is also a sneaky spell.

Medium armors are good with average to high dexterity, and become even better in act 3.

Items would be those of a rogue but with a medium armor. Assassin would probably be my subclass.