Nexus King logs are weird to look at when comparing players on different platforms so I only took a quick look at the first part of the fight.
-He is 4 ilvls higher than you in large stat budget items.
-He generally has higher enrage uptime than you, in the first ~2:41 of the fight before reaching platforms he has ~93% whilst you are at ~80%, this is a massive dps loss.
https://www.warcraftlogs.com/reports/compare/3mx2ykMprNDYaPAj/self?type=auras&fight=17%2C17&source=10%2C14&start=6789927%2C6789927&end=6951527%2C6951527&ability=184362
-You are rampaging after every single Bladestorm and not pressing RB to take advantage of Slaughtering Strikes/Brutal finish.
-You seem to be pressing Rampage too early <115 rage which causes you to miss out on RB casts throughout the fight.
-At ~2:00 when you phase he charges in whilst you just sit afk for 15s doing nothing.
There isn't much difference from 3:43 to 5:44 so I didn't really check anything.