Ok managed to get 3 stars on MoC 12. Think it was more a matter of figuring out the right team for my account and understanding the Aventurine boss mechanics a bit better, because on the run where I beat it I actually did the first half one cycle slower than the team was capable of.
Used Fu Xuan/Sparkle/Silver Wolf/Acheron for side 1 and Ruan Mei/Aventurine/Clara/Jingliu for side 2. (For reference the team that could 1-cycle the first half swapped Fu Xuan for Aventurine with Trend of the Universal Market which gives (E2) Acheron all the ultimates, but I really think I needed Aventurine on side 2)
Initial reactions are Aventurine is very strong. Not really even done building him, but he was able to solo sustain side 2 of this fight relatively comfortably while Fu Xuan wasn't at all, in addition to providing actually reasonable damage of his own. Also provides the team with a CRIT DMG buff and tons of Effect RES, but Fu Xuan does have comparable abilities there at least.
I didn't do guru this year but azuarc can be in my sig anyways.