How can I use string substitution to batch remove leading zeros from filenames? For example, I want to change 2020_001_002 to 2020_1_2.
Within Bridge I do not think so. However, if you are on a Mac you can use the application "A Better Finder Rename" which can use Regular Expressions.
Not sure it helps you but it might get your thoughts going
In it's simplest form, one does not even need a regular expression, a simple find/replace for 00 is all that is required.
Of course, if there are double zero digits in the filename that do need to be retained, then you need to introduce a regular expression based pattern match.