Yeah, cull is always good & it always take 18% off the duration of the fight, which is why the 18% cull is worth 22% more dps (100 / 82) & it has the benefit of reducing the impact of any enrage mechanics by artificially shortening the fight.
From the above, Brittle Bones is worth 22% more dps, Marked for Death gives 30% more. If you have any other way to proc Marked for Death (idols that give minions a chance to apply it or the passive node that gives a chance for Marked for Death on minion “death”) then I’d probably go that route. If you’re using Wraiths, them “dieing” from life decay counts for the passive node that applies Marked for Death & I don’t think there are many nodes that can give you 30% increased dps for 1-2 points (which is all you’ll need if you use Wraiths since they die/time out/etc so often). I tried the passive node & that was up pretty much the entire boss fight, there’ll only be a period at the beginning of the fight that it doesn’t proc Marked For Death, but once it procs the first time, you’ll have enough Wraiths dieing to keep it up for the rest of the fight.