If the first encrption would encrypt the entire drive, then you’ve effectively done a 1-pass erase. It might work, but I would still prefer specialized software

If the first encrption would encrypt the entire drive, then you’ve effectively done a 1-pass erase. It might work, but I would still prefer specialized software — perhaps the encryption method of choice would still leave some data unencrypted or untouched. The hammer method also sounds fine to me.