After another Temenos run on DRK, I stand by my suggestions. Souleater would fill the gab during LR down.
I honestly don't even notice Last Resort ever being down during Limbus or Sortie runs. You're travelling more than fighting, and especially in Limbus, each floor takes like 30-45 seconds and then you're moving to the next camp. Any time lost swinging slower is kind of minimal, plus as /DRG you get a couple of jumps/Absorb TP for an instant refill of TP. By the time I switch to my second target, the floor is done, so Last Resort downtime is practically non-existent from my experience running DRK in either of those events. You may notice it slightly more in Limbus, but never in Sortie.
It wouldn't hurt if they removed the casting penalty on Hasso, I'm not sure it necessary in todays game.
None of the dark magic spells have any real lengthy casting durations; Absorbs and Drains are practically instant, Dread Spikes/Endark is only really used at the start of a floor/fight so does not take away from your attacking. So I'm not sure that's a real issue for DRK's native spells. I'd be in favor of finding a way to make Impact casting significantly faster, perhaps by allowing Consume Mana to instant-cast your next spell or something similar to Spontaneity. OA+Impact is like 1800 TP before SAM roll, but it takes a few seconds to go off. Still probably faster than swinging without Last Resort active, so it's a net positive. But would be nice.
You can supplement loss of Last Resort like I said by rotating those: Jumps (2), Absorb TP, Impact OA. Not great, but faster TP gain when your attack speed is lessened.
It’s nostalgia. It looks cool. Caster DD.
I find very few DRKs treat DRK the way it is designed. They play it almost exactly like WAR and I feel it is underwhelming to do so. Other DD jobs you can get away with pressing one button nonstop until things die and you have to be cognizant of survival, but on DRK, there's a few extra things you can do to make the job fun, and you can afford to take risks because the job is based on that premise.
For example, in Limbus/Sortie/Segments, I almost never see people pumping out Absorb INT/STR/VIT or whatever on mobs prior to fights to boost their own damage (depending on WS). I almost never see people Absorb-Attri on unique buffs, which is practically a free dispel or buff steal. I never see people dropping into a DT+ or non-ilvl set and using Diabolic Eye+Scarlet Delirium for max damage boost potential. You can DSNV Drain 3 and have a ton of HP and then just tank everything (your parse won't display how much damage you absorb, but it still counts). You have a free 100%* stun in Weapon Bash, which can be chained for a free SC (Sortie Boss start). There's tools available to you; yes using some of them
might take away from your dps slightly, but you also gain a huge amount if done right.
The job is still a "high risk, high reward" style, but people play it like a WAR with a great sword or scythe and almost never cast anything, and thats the issue. You can afford to take greater risks and buff your self to a greater degree, but it takes good timing to use all of the tools available.