Nice work! If I had a copy lying around, I would definitely play with your cards.
In terms of weight and importance, that was already 90% of my post. The remaining 10% are my thoughts about how you could improve the wording.
Scrap Metal - Action:
Costs $3
Discard 2 Action cards or 2 Treasure cards. If you do, gain a card equal to their combined costs.
Pedantic wording issue: it's the card's cost, not the card itself, which should be equal to their combined costs.
Suggestion: "gain a card with cost equal to [...]".
Pulley Action
Costs $3
+1 Card
+1 Action
Reveal your hand. Reveal the top card of your deck. If it does not share types with any cards in your hand, put it into your hand. Otherwise put it back or discard it.
If I reveal Fool's Gold and my hand is {Tunnel, Harem}, do I put the Fool's Gold in hand? In other words: what, exactly, does "share types" meanan equal set of types, or just a set of types with a non-empty intersection?
Suggestion: "If it does not share a type with any card in your hand, [...]", in case Tunnel blocks Fool's Gold. Maybe "Put it into your hand unless you have a card in hand which has all the types of the revealed card and vice versa" for the other interpretation. But only maybe, because this looks very wordy.
Iron Melter - Action:
Costs $3
Trash a card from your hand. Gain a differently named card costing the same amount.
If its an
Action +2 Actions
Treasure +$2
Victory +2 Cards
Is "it" the trashed or gained card? I'm 73% confident it's the gained card, but I would like that number to be much higher. (The Ironworks template works for Ironworks because Ironworks only does stuff to a single card.)
Suggestion: "if the [gained/trashed] card is...".
Laborer Action Reaction:
Costs $4
+1 Action
+1$
+1 Buy
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When another player plays at least 4 cards of one type you may discard this from your hand. If you do, gain a Silver on top of your deck.
If I were a pedantic m*f'er, I would point out that you never play 4 cards, you always play your cards one at a time, so the reaction can never trigger

Less tongue-in-cheek, suppose another player plays: Village, Spy, Spy, Minion, Relic, Silver, Counterfeit trashing Copper.
After which play may I react? I'm thinking Minion (4th action), Relic (4th attack) and Copper, 1st play (4th play of a treasure).
The template "when <condition> is true, do <effect>" has problems. I think "when <event>, if <condition>, do <effect>" is better.
In this particular example, I suggest "When another player plays a card, if they have at least 4 cards in play which have a type in common, <effect>" or "When another player plays a card, if they have played at least 4 cards this turn which have a type in common, <effect>".
Tracking is much easier with the first template. The difference may matter for King's Court (1+3 plays, 2 cards in play) and Embargo (1 play, 0 cards in play), among many other edge cases. I think the number of plays will always be at least as large as the number of cards in playand I can't wait for people to tell me how wrong I am

Tax Treasure:
Costs $4
Worth $1
You may trash a treasure card from your hand. Gain an action card costing up to $2 more.
If you choose not to trash a card, you gain an action card costing up to $2 more than... what, exactly?
Suggestion: "[...] hand. If you do, gain [...]".
Also, since it is different from the Tax from Empires, giving it a new name would make the sets more compatible. I don't think anything breaks if a card and a card-shaped object have a name in common, but you never know

Paper Boy Action Attack:
Costs 4$
+$2
Each other player with 5 or more cards in hand reveals the top card of their deck, discards it, and places a card from their hand that does not share types with the discarded card on top of their deck. If all cards share one type, they reveal their hand put a card back on top of their deck.
For accountability purposes, if they don't reveal their hand they should reveal the card they topdeck.
Just like earlier, I still think "share types" is unclear.
Lead Figurine Action:
Costs $5
Discard any number of cards. +$1 per different card type discarded.
---------------
When you gain this, immediately have a buy phase with +$2 and +1 Buy in which you may use treasure cards that were used to buy this card. This cannot cause you to have more than two consecutive buy phases.
I don't think the rules define what
using treasure cards means; I suppose you meant play.
The rules also don't define what "using a treasure card to buy a card" means. Suppose I play Counterfeit (trashing nothing), Gold, Gold, Copper, then buy a Silver and this. Am I using Counterfeit to buy this? I could buy it just off of 2xGold, but only if I don't buy a Silver first (which I do) unless I have more +buy from some other source.
Suppose I Crown a Woodcutter, am I using Crown for my second and third buy this turn? Am I using it for the first buy? Does it depend on whether I need the money from the second play of Woodcutter for my first buy?
Suppose I play Silver, and Counterfeit trashing Copper; am I using the Copper to buy this? If I choose to (re)use the Copper, do I play it from the trash, and then put it in my discard pile in my next clean-up phase?
Protest Action Attack:
Costs $5
+$2
Each other player with 5 or more cards in hand reveals two cards that do not share types and discards them. If all their cards share types, they reveal their hand and discard two cards.
"share types" vs. "have a type in common" again.
And that's all. Let me reiterate: nice work, would definitely play with these if I had them lying around.