The idea is a new mob similar to villagers, the caveman. They will have a smaller forehead, smaller nose and wear a tattered cloth coat reaching to their knees. They can be found in ice spikes and you have to thaw them out. You can find 5 "biome" types, cold ocean, warm ocean, below sea level, below hight level 0, and one that is its natural varrient similar to how plains villagers are. This is because of the water ruins, trail ruins, and ancient cities. They will be scared of fire similar to how piglins are scared of soul fire and creepers. They can sell you long forgotten things but with a different currency than emeralds. They will sell you items like copper tools, this will be the only way to get them. They will make pottery shards if you give them clay. They will give forgotten enchants known to modern enchantment tables, a battle hammer that can't be crafted. And more things forgotten to time.
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).
The reason I didn't want to make them a different creature is because these are the ancestors to villagers, they look similar because of this. They give you copper tools and other uncraftable items is because you won't know the recipes they are lost to time. They give you this as a lore stand point. Because real ancient humanoids had copper tools, and copper tools is a frequently posted suggestion and I'm offering it to them. The reason why they don't accept emeralds is because if you time traveled to 1980 you couldn't take a 2000s currency because they'll think its fake. The player would want to thaw them out to get these uncraftables or new enchants because you can't get these conventionally in the modern times the game is set in. People also want renewable pottery shards, if you give them the shard they will replicate then them. It also is lore to the previous people who might know or were the ones who made these ancient structures. If you don't want to interact with them like the sniffers you can avoid them entirely. But since the battle hammer is like the trident someone will want it, these things will be more powerful as a reason to go out and thaw them.
I'm not really sold on the ancestors of villagers being underground in random ice spikes and needing to be thawed out. If you want to do some villager ancestry thing, I feel like it'd make more sense to do that by referencing them instead of having them as mobs stuck in ice. You could have abandoned, ruined structures that resemble the villagers' building style, with archaeological loot that strongly hints to them being villagers or at least related to them.
As for the new items, "but the lore" and "people post the idea a lot" isn't really a good justification for adding these as trades. Let's take copper tools for example.
Firstly, people aren't going to trade stuff for pointless tools. If copper tools were some neat antique you could find in archaeological sites, that'd be better—there wouldn't really be a cost to them, and it'd be clearer that they exist more for lore and less to be useful. However, if you have to trade for a unique item, the player is going to expect the trade to be useful. By the time the player ever finds one of these guys, they're going to be well beyond the point that a copper tool could be helpful.
Secondly, one of the reasons people want copper tools so that copper has more uses. Granted, I don't think this is a very good reason for wanting copper tools—you use very little copper on them, and they'll be replaced by iron in minutes, so it's not like you'll get a lot of use out of all the copper in the world with these tools—but if your argument is going to be "this should be added because people want it", you need to consider why they want it.
Elaborating on the second one, let's take renewable pottery shards. Yes, people would love pottery shards to be renewable, but do you think they want to have to go underground, look blindly for a rare frozen mob, thaw it out, and then come back and trade with it whenever you want to renew pottery shards? Pottery shard renewability should be reasonably accessible, so this isn't a good way of implementing it imo.
When you're adding new items and whatnot, you need to think about their place in the game. What do they do? What are they trying to accomplish? Is it the best way of accomplishing that? A battle hammer seems like it could a really cool melee weapon, but I have no idea what it does that separates it from other melee weapons and makes it worth seeking out.