Please look up the definition of NP-hardness again. If the problem (#) := "finding a nonce that makes the block have a SHA256 with many leading zeroes" was NP

Please look up the definition of NP-hardness again. If the problem (#) := "finding a nonce that makes the block have a SHA256 with many leading zeroes" was NP-hard, it would be an interesting idea to use a solver for (#) to solve e.g. SAT (not the other way round!). TLDR: You talked about the wrong inclusion.