I've been running a dark tank for awhile now (he's level 40 something) and I've had a 50 dark/dark brute in the past. I wrote a lot more than I thought I would, and in no way do I consider myself an "expert opinion", but here are some (a lot) of thoughts.
Zephyr will work when power isn't on.
A good solo combo should always include Hack as the first move because of bruising. It's actually pretty viable to just hack something to death, since it's a decent ST move.
I wouldn't recommend even having Slash, and instead just let Death Shroud eat away at the enemy. My reason being is that Slash (going by base damage here) does around 45 Lethal damage while Death Shroud does 8 Negative damage. Slash takes 1.5 secs to activate and 5 end to use, while Death Shroud consumes .5 end a second to do 8 damage to at most 10 targets around you. So with base numbers, Slash takes up a total of 5.5 seconds(recharge + activation) and 5 end to do the same amount of damage Death Shroud does while waiting for Slash to recharge, but it can affect up to 10 enemies. Noted, Slash does have a higher accuracy than Death Shroud, but DA does Negative damage, which is resisted far less than Lethal is. With enhancements, Slash could be worth it, but if you're having end issues, this would be a power you might reconsider.
First, you should always have a team. Tankers aren't the biggest damage dealers, but they sure can take it better than most and if it's your team, you decide the flow of the team. If it's not your team, you should either share aggro with another tanker entity (tanker/brute/illusion control) or engage carefully. Plus, you get more xp with more people and it's not just because you're killing more enemies that are stronger. Dark Armor, above all else imo, should have a team.
From what you say in your post, you use Dark Regeneration near the beginning of the fight when it's the best heal in the game with 10 targets max and 30% heal on one when it's unenhanced. This should be used when you're near death because then the enemies will have to hit you as much as when you first jumped into the group. Usually, the alpha strike will do this to you, so I understand if you use it because of that. This is similar to Fire Armors Healing Flames, except it's much more powerful, but requires hitting a target and it's probably the thing draining all your end (30 end if unenhanced). This power shouldn't be used for small bruises, but for the worst wounds. I would actually drag the damage from one group, get hit by another one a few times, and then use it.
Your combo should include taunting frequently and using parry a lot. I don't know what was going on with your taunt when you said it wasn't working, but it's auto hit and will work up to 5 targets max. It's possible somebody else had a much longer taunt duration in effect on it, or you reached aggro cap (15 or 17 peeps). Taunt doesn't fully pull people to you, it gives them a -75% range penalty, so all but the longest range people (Council and Nemesis snipers usually) will first use their ranged move on you before coming over to use melee. It also costs 0 end to use, which helps since everything costs so much. Damage is a factor in how much enemies will pay attention to you, but taunt is weighed much more heavily. Someone would have to deal, very roughly, 10 times the damage to equal taunt + how long it lasts. Taunts from other people have to be more than half of your current taunt duration to steal the attention away (taunt auras usually do about 13 secs of taunt, so basically any other taunt will steal that away).
Parry gives you +15% defense to melee and lethal, which can boost your survivability immensely if you stack it twice. This also works really well with Cloak of Darkness (for some defense) and Cloak of Fear since it has a -5% to hit to the targets it hits as well as fearing them. This should be your bread and your dark regen would be your butter.
On my dark tanker, I almost never attack. I taunt the hell outta everything, and use my first power (in your case, hack) since tankers apply bruising with it. That's a -20% resist debuff on the thing you hit it with. I use my auras to taunt everything I didn't hit with taunt, but I'm probably an outlier. I took both Oppressive Gloom and Cloak of Fear and I like how it stuns most minions and fears the lieus. It takes a bit longer for it to work on bosses, but it will if given the time.
As for enemies, Dark Armor is unique since it kinda works against most things;however, DA won't excel at anything but Psionic. Energy and Toxic are its biggest weakness. Energy is fairly common, but a lot of bosses and some AVs use Psionic, so DA is pretty good late game.
Soloing would depend on the diff. Tanks can definitely handle +0/x0. +1/x2 might be the most I would recommend, otherwise you'll be without end a lot if you had too many enemies. If anything, xN is better than +N, since you can usually AoE things to death with Shroud + BS moves. If you've got the money and Catalysts, try buying a few procs to put into Death Shroud so it pays to have it on, but in your posts, you say you don't have much. I would recommend collectingreward merits and trading those for desperately needed kb protec IOs.
The way I play is I run around within the enemy group, letting my auras mez them. Oppressive Gloom will stun minions, which make up the majority of a group, and Cloak of Fear fears lieus. I aggressively use Taunt on anything that looks like it isn't looking at me. If I had /Broadsword, I would use parry a few times before using much else. If it looks like the team can kill, I won't use any AoE since DA needs to conserve end.
Dark Armors, imo, are very passive. They're there to get hit and annoy the enemy, and just when the enemy thinks they've got you, you heal back to full. Oppressive Gloom and Cloak of fear are less mezzes and more mitigation. DA doesn't have much else to buff itself, but with that sword of yours, you've got a lot of potential. Sure, it might suck now, but with enough time and practice, you'll be swallowing(?) the enemy.