Spliff
Godzilla got busy.
What is the magical number/threshold to distinguish the rep ranges between high and low volume? Is it subjective?
It's not the rep ranges in that routine that's the problem. 6-8 reps tends to be ~80% of max effort which is where muscle fiber recruitment maxes out. Pretty standard.
The frequency along the intensity required for strict 6-8 reps + rest/pauses and drop sets will burn you out fairly quick
Pay attention to how often the shoulder gets hit when looking for routines. Rotator cuff stabilizes in every direction. It's important to give it a rest.