As a free alternative to Flash Renamer, PowerRename[0] from the latest PowerToys[1] collection might be worth a look.
[0] https://docs.microsoft.com/en-us/windows/powertoys/powerrena...
[0] https://docs.microsoft.com/en-us/windows/powertoys/powerrena...
However, when there's directories and many, many files involved, I just use Python with pathlib. It's a couple lines of code anyone can learn to do, with all the flexibility of Python. PowerShell would maybe also be suitable.