Did you not notice the stated requirements are different between the 3 monoliths?
Spirits of Fire says it requires 50/350/650 or 150/450/900 empowered. Last Ruin says it requires 50/300/650 or 150/550/900 & Age of Winter says it requires 50/350/650 or 150/550/900.
While this is a little bit inconsistent, since the total requirement for all 3 echoes is the same, this doesn’t really impact the total length of the timeline.
And since people complaint about the total time required for killing the boss, this is not that important.
True, but it’s inconsistent for no good reason.