Isn’t there a msitake with your proof of work?

Because the way you implemented the proof of work, it isn’t tied to the transaction history.

This means that somebody could re-write the transaction history, compute the new block hashes, and re-use your proofs of work.

He could then mine a single new block, and thus create a new longest chain and cause everybody to accept this chain.

The way to fix this would be to include the hash of the last block in the proof of work for the next block. This way, proofs of work can not be recycled on other chains.