While I'd personally go with SSH for sftp/scp, a bare-bones, mostly stock solution would be FTP.
The toggle chain is part of NickelMenu's examples, and it doesn't require anything extra (except FW >= 4.22 to avoid old busybox issues w/ ftp).
As for a macOS client, I recall using something with a duck in the name or icon at some point, but damned if I can remember the full name ;D.
EDIT: Yay, Google brain: it's called Cyberduck. I have no idea what actual macOS users usually prefer, though

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