You have to construct a raw transaction and then hash it twice to get Z.

This link shows you how to do that, follow the steps up to step 14: https://stackoverflow.com/questions/60255876/signing-a-bitcoin-segwit-transaction

Is this a P2PKH (1.....) address? Because the script sure looks like it and you also have no witness data.

Just asking, because no Segwit wallet I know of had a nonce vulnerability that RSZ breakers rely on to work properly.