Hi
I calculated which dungeon gives you how many Echos per hour on +5, +10 and +15 with the options to +1 it, +2 or +3 it.
There might be some mistake but I hope this is useful
This table shows Echoes gained per hour if you constantly spam the dungeon. The amount of Echos you gain does not change based on the dungeon or the timer.
You receive 15 echoes per level. Therefore:
75 for a +5
150 for a +10
225 for a +15
But the dungeons have different timers. So farming certain dungeons is faster than farming others. Also, your confidence on +++ing or ++ing a specific dungeon might influence your choice on which one to spam.
TL;DR
Do Atal, Workshop, Freehold, Underrot and Siege
Avoid SotS, King’s Rest, WCM and Motherlode
Doing an Atal+5 +++ is more efficient than a SotS+10 +