Debuffs will rarely be outright reliable, they’d be broken if they did considering how much they swing the turn economy.
That said, boosting spell save DC goes a long way (either by primary stat or gear) and gets pretty strong by late game. Upcasting gives you more targets to try your luck on, too. Early game you’re bound to have some misses just like your martial fighters do, but Sleep does very well then, as does Phalar Aluve: Shriek.
In general, having options that check against different saves and being smart about your targets helps a lot. Don’t try to throw Hold Person on an enemy druid or cleric, and use Bane on low CHA targets like undead and constructs, for example.
Yeah, it’s all equipment plus Elixir of Viciousness. There’s four pieces of gear with an unconditional crit threat range boost, and a cloak that tacks on another if you’re hiding, but I can’t be bothered to do that, hah. 14-20 is good enough.
With the cloak, Dark Urge can get down to 11-20 at the end of the game.
So if you’re in the Underdark, you’re not actually in Act 2 yet. If you need the main story to drive you forward, you have a little further to go. The Underdark doesn’t advance any personal quests, at least not in a major way, even though there’s plenty to do.
The difference between this and D:OS2 is that while Reaper’s Coast felt open, the level scaling still had the zone on rails, more or less. Here, you have a couple different ways to go. If you want to move on (I suggest level 5 if you aren’t already first), there is a way forward to find. If you want a more straightforward road, try the path on the surface.
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).