Firstly, "villager but underground & different currency" just doesn't seem like all that interesting of a concept. From what I can tell, they look similar to villagers and are interacted with in the same way. Piglins are a good way to go about a new trader/civilization mob: completely separate from other civilizations like villagers and interacted with differently. You already have this whole thing with thawing, which would fit a more nonhuman mob, so why not come up with something unique? Maybe a golem, something related to mushrooms, or something super weird and new like endermen.
Secondly, you've mentioned copper tools, new enchantments, and a battle hammer, but what would these actually do? What justified them being locked behind finding and thawing an underground villager? This is more of an issue with how much detail there is in this post than the idea itself, but even then, I can't think of anything copper tools could do that would justify them being a trade from these guys (or that would justify them existing at all tbh, but that's a separate discussion).
ThatOneKirbyMain2568