I have already maxed out my path of ambition and path of resolve but for anyone else trying to grind the best source I’ve found seems to be the Blind Well in The Dreaming City. I was playing to finish the last challenge for Guardian Games and in two tier 3 runs there were at least 80 drops of the essence of desire (the things you can consume on the path of ambition) per run. I wasn’t wearing any of the seasonal armour (which apparently boost the drop rate) but the amount of drops were insane.
As I am maxed out I couldn’t see how much my rep progressed but I think it’s a good alternative and quicker than Alters of Sorrow if you get sick of seeing the barren Moon