I would suggest either:

The Secure-delete suite of tools

Or BleachBit

These articles provide a good overview of each one and their use:

https://www.unixmen.com/securely-delete-hard-drive-data-with-secure-delete/

https://ssd.eff.org/en/module/how-delete-your-data-securely-linux

I also strongly recommend reading this section of the BleachBit Docs for an excellent write-up on properly securing/deleting data in general... in short, for your use-case:

1. Shred the file/s ('srm' in secure-delete).
2. Overwrite all free disk space ('sfill' in secure-delete).
3. Ideally encrypt the data (e.g. credentials could be stored in a passwors keeper, like KeePassXC) and/or the entire drive/volume in future.
4. And in the end, mechanically destroy or degauss the drive (if it's that important to you).

Hope that helps.