I don’t want to undermine the premise of the thread. However, Mammon’s lust for gold is due to his nature as the embodiment of greed. The gold itself is immaterial beyond that it is the most tangible measure of wealth. The value of gold is based on its properties (bright, shiny, untarnishable, malleable) that’s why it’s been seen as a sign of status across so many different cultures. Mammon represents the accumulation of wealth and lives on an infinite plane. He collects it to have it, not because there is some practical or industrial use for it… that would undermine his concept as a creature of greed.
That aside, if you want to have gold be something more than just wealth, I would have it be the literal ‘root of all evil’. The matter of the nine hells condensed under immense pressure - like coal. A great event in the earliest days of the multiverse (Perhaps the fall of Asmodeus) spread it across the material plane and laced it in between all the other matter. It also explains why evil exists in the material plane.
Silver is the condensed matter of the upper planes - which is why it has an effect on devils and werewolves etc. The same event saw silver spread across the material plane.
Copper is the condensed matter of the transitional planes - where the planes meet, or embody neutrality making it the most plentiful, and least valuable. But best able to alloy with other metals to form zinc, bronze etc.
I would change upper planar creatures solar, devas etc to be hurt by gold weapons.
Dragons are able to derive pleasure or relaxation from this fundamental material. Perhaps it sustains their magical nature or breath weapons. You can have chromatic dragons favour gold and metallic dragons favour silver.
Mammon might be collecting gold because he is trying to accumulate enough to build his own realm/lower plane separate from the Nine Hells and out from under the thumb of Asmodeus. It just appears to makes him look like he’s greedy. He’s really interested in self preservation.
Yugoloths are aware of the nature of gold (as the original fiends, the first to crawl from the grey wastes of Hades) and they see it as their sacred duty to reclaim the stuff of the lower planes scattered across the multiverse. Gold is also an integral part of transforming a Yugoloth into an Altraloth. One of their greatest deceptions is that they want gold to hoard rather than for its true reason. Mammon has uncovered the secrets of the Yugoloth, possibly by striking a bargain with one of them.
(This is inspired in part by Minecraft, where the Nether - their equivalent of hell - if is filled with gold seams, and gold collecting creatures)