Light Metal -> Light Metal -> Weak Armor
Essentially, the idea is that as a lightweight metal, gold is also fragile. The reason we don't ever see golden weapons outside of enchanted ones in fiction like Thundercats? It's weak. Having an entire casing of a weak metal sounds to me like the transition from Light Metal into Weak Armor makes all the sense in the world.
Illusion -> Shield Dust -> Illusion
This one might vary in reception, depending on what we end up having as our artwork. The logic with this goes under the assumption that we get a more cocoon-esque second stage. The larval form must camouflage to survive until it reaches a stronger, more mature form. Likewise, Aurumoth itself uses Illusion as a method of survival and deception to its prey. This leaves the middle form, leaving its stage as prey and entering into predator territory, which comes off to me as passive-aggressive. If a more cocoon-like Pokemon is supposed to be a predator in only some scenarios, wouldn't it make sense if it was defensively inclined? As such, the best natural defense comes less from hiding and more of standing up to other predators. Hence, Shield Dust comes straight to mind.
I don't have any ideas for No Guard's transition, but I thought I would help by listing the former two ability lines. Thoughts?