if bash/sh scripts get too long (or long-lived), I switch to python when possible.

As a minor example, I hate having to remember (or explain) ${var##pattern}

It is useful for manipulating paths, but it is cryptic.

in comparison, something like:

  newfile = os.path.join(newpath, os.path.basename(oldpath)) 

Also, argparse is the best.

That's because you don't use the native lib pathlib from python ! It was a delivery when I knew about it. For example, to join paths, you can do so much easily, see the section "operators" of this link : https://docs.python.org/3/library/pathlib.html