Just from looking at your longest attempt, your uptime is fairly low on your dots.
pretty much this. there are 5 spawns on either side of the room where the little adds come from. not sure if your dotters are assigned a "lane" but if so try to get the middle one, or at least one off from the middle. youll be able to reach 4 adds with very little movement, and they spawn about every 8 seconds. with minimal effort you can reach all 5 + the large add(s) the tanks have and dots everywhere.
are you using a nameplate addon where you can see dots? this is very helpful too
Soul Burn: Soul Swap on as many adds as you can, from the opposite side of the boss (max travel distance for the adds = maximum time to kill them). Looks like you have several dotters, so you're doing the dot-killing tactic. Meaning Destruction would be useless.
Back when destruction was bugged and adds affected by immolate took 100% more damage from all other warlock spells on that fight, that was the way to go. But that only lasted for a few weeks. Affliction is definately better than destruction on the fight, especially with what seems to be your tactic.
Just dot as many adds as you can (3-5 lanes) and replenish your soulshards by executing low adds as often as possible to keep the SB:SS's going. I pull between 400k and 500k dps on the fight, depending on how fast I get the stacks and what stacks I get etc. Sub 100k dps means bad play and your dot-uptimes reflect this. Atleast on the one log I checked.