There are limited ways to protect an Excel file using the Python package [ openpyxl](https://openpyxl.readthedocs.io/en/stable/protection.html). But it would only offer protection from some changes (not from opening the file) and the password would be stored hashed - so no real hurdle there.
In general I think most password protections for Office files are more there to prevent accidental editing to keep a fixed structure - but they are insufficient for real protection.
Please test the Code first. On a Mac I encountered some problems with the sample files handled thru the Python code, although it did work. So before encoding all your files test it with a copy and always use backups.