It is hard to determine ‘hardness’ of a given workout - there are so many variables. It is similar to defining pain in a clinical sense - pain is very subjective and varies greatly per person.
I generally have pretty good muscle endurance, but here is how sweet spot feels to me and what my HR and perceived exertion are.
My max HR is in the low 180’s, the max I hit inside on the trainer is usually around the high 170’s. What I hit on my latest ramp test was 176. If I am doing say…Tallac - my HR will stabilize in the high 140’s or low 150’s for the first interval, the low to mid 150’s for the second, and then stay at the mid to high 150’s for any future intervals (count of intervals depends on which version of Tallac I’m doing).
All of the intervals feel very mentally taxing to me, but I don’t start to feel heavy muscle fatigue until I’m into the second interval and even then the fatigue typically starts to build only after 5 to 10 minutes of the interval. Consistently the first few minutes of every interval feel easy as my HR is low
In my experience sweet spot should be as much of a mental challenge as a physical one