Akvan / Kaggen / Pil will drop single metals about 1 in 3 times which can feel a bit more satisfying and consistent, but over the long run gives less metal per stone+time spent.
I'd suggest sticking with Qilin / Aello / Uptala if you are after metal though and accept its really streaky and focus on your long term average per 100 kills.

In general focused farming for RMEs for its specific currency to the exclusion of other activities to finish it has never been really advisable, and its a recipe for burning out on the game.

All my mythics for example were funded with drops from doing dynamis and voidwatch and selling those drops to buy alexandrite to supplement the progress from also running salvage.
When I made my 119 armageddon I used dynamis+salvage drops to fund the dross+hmp acquisition , and maintain a variety of activities to avoid burning out.