Right now, I got strengthening at level 64, which costs me around 110k per item/level.
I also unlocked Eight Inner gates recently, where the available gates are filled with a few yellows, mostly purples and a couple of blues.

Anyone got tips on how or where I can spend the leftover coins for best power per coin in return?

(always maxing group donations + keeping a fixed amount for black market if something pops up).