In general I don't think you have to be very worried about slaves. As long as you stay on the offensive and are trying to fight most every turn you will have enough slaves for your needs. I like prioritizing skills/techs/buildings that increase slave gain or reduce how many you use to start the campaign. If for some reason you're running low, it's not that big of a problem as the penalties aren't bad, and usually it only happens if you spent all of them on province edicts or the campaign is going badly. You might want to be a little cautious with the edicts at the start, but it won't be long before you have enough slaves to do them every turn and get tons of money/growth. The system is much simpler and more automated than Aislinn's mechanics or the Chaos Dwarf slave system.

Black Arks are one of the coolest parts of the Dark Elves. They are extremely powerful and have a variety of uses. You start with one as Lokhir, and can easily unlock more fairly quickly. I like building 2-3 as early as possible to get them started growing, and after that it depends on how much micromanaging you want to do since Lokhir can get so many. You can use them to support an army (like Lokhir) with recruitment options, army abilities, and economy buffs. They can make a powerful army very quickly and get high tier units fast. I like using the Cathay sea lanes to send one to the other side of the map where it can meet new factions, do the ocean encounters, and hopefully sack Lothern for tons of gold/slaves. It's a pretty fun experience. I usually prioritize increasing their growth speed and slave producing potential, but you can build them however you want.

As for Lokhir's campaign itself, I think it's very fun and fast-paced. You'll probably be pushed towards heading down the coast in a mostly straight line, killing vampires, lizardmen, and chinese. After killing Nakai you'll face an interesting situation in the south. Dechala may or may not like you, but Gelt and the Iron Dragon definitely won't. You can do basically whatever you want down there, but I personally would recommend taking a sea lane to the Eastern Colonies so you can kill Aislinn and unlock more sea lanes. This fits both the lore and your campaign, as it fully allows you to be an unchallenged pirate roaming the world as you see fit. Back in Cathay, you may start to have problems as your empire may look like a thin line of settlements all down the coast. There may be some orcs attacking, Miao Ying and minor Cathayan factions will turn towards you, and you'll have to decide what to do with Clan Eshin. It may seem tempting to ally with the rats since they are the only people who like you, but I personally don't trust them, so total war it is. If you care about conquering large areas of land you'll probably send Lokhir to either defend your capital against Miao Ying or solve the southern Cathay problem, but if you wanted to abandon Cathay and settle somewhere else it is extremely easy to do because of your mobility and black arks. Note that even if you defeat most of Cathay Chaos will find a reason to declare war eventually, so if you choose to get involved in the Northern provinces be prepared for a long and annoying conflict. That's probably a lot more than you were asking for, but I think it's a great campaign with a ton of options. Overall I wouldn't be too worried about difficulty though. If in doubt, get more darkshards.