The best source for Stepmania charts is Zenius-I-Vanisher. Unfortunately, there's no easy way to incrementally update collections.
To run this script you'll need Python 3.6 or later, and you'll need to install Beautiful Soup with e.g.
pip install beautifulsoup4. Put it into your Stepmania folder and run it with e.g.
By default it will download the latest 20 official SIMfiles but you can give it one or more alternate URLs to pull from, for example:
python3 zenius.py https://zenius-i-vanisher.com/v5.2/simfiles.php?category=top_official https://zenius-i-vanisher.com/v5.2/viewsimfilecategory.php?categoryid=41
ZIV is an amazing resource. Don't abuse this tool, and consider donating to them to keep it running!