You might be able to do all or at least the bulk with BIMP
http://www.alessandrofrancesconi.it/projects/bimp/You would not need a new layer since you would save to a different folder and/or rename and Bimp will do the flattening.
In Bimp (File -> Batch Image Manipulation) just keep adding operations, then apply to the list of files.
Here is a composite screenshot that might give some hints. It will often, but not always pick up other external scripts and plugins so not always limited to basic Gimp.
Attachment:
bimp.jpg [ 260.98 KiB | Viewed 2742 times ]