Your fancy new dark hand blocks 80% of magic, fire, and lightning damage, very useful against mages if you use it as a second shield. Otherwise, just get better at dodging spells. I never get hit anymore by sorcery unless it's a sneak attack or in a gank, and only occasionally catch a Great Combustion or Great fireball (Sometimes I forget the actual thrown fireball does damage so i try to roll directly at them and through the actual ball they throw and get tagged)
Edit: In terms of weapon choices, The Clay and halberd are both great weapons, the clay is beginner friendly but the Hal takes a lot of practice and timing to use to its full extent, though when you do it's incredible. It doesn't do well against turtles though unless you can dead angle with the R2s so carry a backup that's more suited to fighting those type of people. The Gargoyle halberd has really good AR but not much scaling, so it's better off as an elemental in most cases. It's another great weapon with the Black Knight Halberd's r1 moveset. I'm also a big fan of spears, especially as a secondary weapon, but again they need practice to become great weapons. I can pm you a literal treatise on spear wielding if you're interested, btw.
For when you get to 40/40, I would try out the Murakumo. It requires 28 str and scales with dex so it's a demanding weapon to use to its full potential but if you keep it as a +15 its physical AR, moveset, range, and stunlocking ability are unmatched. If DkS was balanced like a fighting game, it would certainly be considered a high-tier weapon.
At 40/40, the Washing Pole is my recommendation as a katana, as the scaling looks like crap initially, but it ends up more powerful than other katanas at 40/40 with far better range. Some people prefer the Iaito's moveset though, and the Chaos Blade does more damage if you have 10 humanity as well as having a purportedly better bleed effect (I don't have solid evidence either way) but a less viable moveset and a self-damage effect. Power Within, as stated above, is a good buff, but regardless of your vit will drain all your HP in about 60 seconds with a base flame. it's much better with a stronger weapon, as a 200 damage hit will have less of a bonus from a 40% increase than a 500 damage one, unlike the magic buffs which add a set AR, greatly benefitting faster weapons more than slower ones.