You should be able to just use ssh/sftp. There are lots of great clients, and you can absolutely still use usernames and passwords, no public/private key stuff required. You can even use ssh and scp right from powershell on Windows boxen if you're so inclined. There's winscp, and if you want filesystem mounting, there's this: https://github.com/winfsp/sshfs-win
For macos and Linux, the options are far more plentiful.
Edit: there's also file pizza, which is a file transfer thingy with no middle man that's open source, although it's not copyleft AFAICT: https://github.com/kern/filepizza
and similar tools. Not really what you're after, I just think it's neat.