The game seems to know to give the mirror last. So you need the other 28 moetifacts before it will appear either in moetifact gacha (which you shouldn't be doing) or the "buy next moetifact."
This is because the mirror is the best one, 5 less normal monsters per round makes runs faster which leads to faster crystals.
My question is when does the Codex reach max level? I'm hoping 200 because im at lv142 and its costing 14k+ crystals per level