MuWz Kiku -> Ash

That's the complete sequence.  MuWz switches to Ash as soon as he receives the Necronomicon.
Ash is key because Clarity, Mu's key weakness.  Also, Ash fixes Mu's terrible apts.  It allows Mu to be good at one thing, whatever weapon he's holding.  XP xfer gives slow flexibility in response to gear changes.  Mu has lots of innate immunities, so doesn't mind cursing all jewelry for the spellpower boost.  Freedom from exploration traps allows MuWz to cheese forever without fearing depositioning.

Even after hitting skill cap, negating Ash's skill boni, it's still worth staying Ash to abuse Crystal Ball with Clarity for infinite MP.

By contrast, OpVM Ash never fully binds to keep his UC.  Which argues against Ash.  OpVM loves Statue/Lichform UC.  Which god best suits that tactic?  Usk amplifies it, granting pseudo-cleave.  Nem's OK.  Oka works well, surprisingly.  But the logical choice is Usk, for UC cleave and Statuepode enhanced mobility.

Hm, but since Op has Inv 1 Evo 1, Ash does let Op stop investing in Inv...

So OpVM Kiku -> Fedhas -> Zin/Jiyva/none? -> Ash/Usk

If OpVM wants to use Lichform safely, needs Ash's clarity.  Also, Usk competes too much for OpVM's MP.  Ash it is.

Ash works well enough in the Abyss.  OpVM can likely use stealth and poison vape to handle Shining Eyes.  Still, why not go Zin while burning off Fedhas wrath in Hell:1 or Slime?

So OpVM Kiku -> Fedhas -> Zin -> Ash