I’ve read how S3 passwords are saved in plain text, but linux is always being talked about.
Windows is usually poorly protected in small businesses.
How do you recommend I secure these plaintext passwords?
What exactly do you want to secure them from? What threat?
E.g. if you worry that your laptop is stolen and somebody can retrieve them use full disk encryption.
If it is small business and you do not want all your employees to know S3 credentials you can go kopia server route.