what's the use of the sftp-access entry in the shared folders privileges if at the end the privileges are to be set per user?
You do realize that the shared folder privileges are an OMV dialog not something added by the sftp plugin? group privileges can be used with other plugins like samba. They just aren't used with the sftp plugin.
So, yes, it seems super simple to click the sftp-access group checkbox and expect the plugin to do the rest but it doesn't work that way nor do I want to try and code that nightmare.