Remove Files from Items and Subitems' File Gallery when Duplicate Board with Items

Description

We have a board set up for requesting files from others. We need to do this multiple times a year, so when we rollforward the board for the next cycle, we will duplicate the board and the items without updates to start fresh again. However, files are still attached in the file gallery after we duplicated the board in the items and subitems. Is there an easier way to remove all the files in the file gallery?
We have approx. 200 items (not including subitems yet) in the board and we don’t want to go into all the items & subitems one-by-one and manually remove files from their galleries. Each item or subitem, people usually upload a lot of files (usually more than 10 files, some will have 80+ files).
We have try the following: 1. Create managed template and use the template for duplication, but because the board has a lot of items, the time takes to create and update the managed template takes a long time (more than half hour), 2. Automation only clear file columns but not from galleries.

What are you trying to achieve

I am trying to rollforward the board with items only and start fresh again with no files attached to file galleries. I want to remove files from items and subitem’s file gallery easily without so much manual work (i.e. going into each items and subitems, and manually click on the “delete” for each files).