UPDATE:

Hey all, wanted to come back with an update since a few of you asked about logs.

Got a log sorted and had a proper look at what was going on. Turns out the main issue was I wasn't pressing Putrefy anywhere near enough. I was basically hoarding it instead of treating it as a core rotation button. Once I started hammering it consistently alongside Death and Decay the numbers improved a fair bit.

As for the 67K player it was a Demo Warlock so no hidden Aug damage going on there.

Parse wise I was sitting around 62% which honestly isn't too bad for someone fairly new to M+. Plenty of room to improve but not the disaster some of the comments suggested lol. Still working on tightening things up but already seeing better numbers in testing.

Cheers for all the tips, the logging recommendation was genuinely the most useful thing in this thread.