You dont need to make a complicated batch/bash script, its simpler and more reliably to use .lftp own scripting

as a added bonus it simpler to port to other systems
there is some chevats with ssh keys

  1. You have to run it with bash bash.exe -c "lftp.exe -f lftp_mirror_test.lftp"
  2. The dummy password is ignored but it must be in the script

Mirror from server (Download)

Mirror to server (upload)