qBittorrent-nox is as separate as you can get. It can be run on the server without GUI. It also provides WebUI, that can be modified if you wish, but you don’t have to use it. Moreover, most (if not all) settings can be edited through the text file and torrents added using CLI.
qBittorrent, when Web-UI is enabled (doesn’t matter if it’s nox or not), provides and API too. IF you have a client you can connect and manage everything using it. However, the only clients I know are for android.
But that’s the literal point of WebUI of any program. To make it possible to connect and manage the program from any device from the remote machine. And there is a reason why most developers choose to use WebUI - you don’t have to create multiple cleants for every OS and every internet connected device has a web browser. In fact web browser can be considered the client in this scenario.
What could be the reason why anyone could be against WebUI? I can only think of one advantage of native client that has to be installed is that it would make of possible to associate torrent files and magnet links with it.
And lastly, if you would search for qbittorent remote client most likely in top 5 results you would find electorrent. I haven’t tried it but at least description claims that it can be used with qBittorrent.
Edit: heh it seems I managed to reply to my own comment.
Separate qBittorent admin container? angielski
Is there any way to separate the UI from the actual torrenting?