I suppose it’s a bit of a “hoop,” but what I use to download files with curl, aria2, or wget is a Firefox add-on “cliget” (I’m not sure if there’s a Chrome version). You click a link in the browser to download, and it generates a command for these programs to emulate a browser download. I don’t use it all the time, but I do for large files or slow downloads. Since these programs are CLI-based, I can run them on my server and let them work for as long as needed.

I guess it’s still not an option for people who download a lot using direct downloads, but still just putting it out there as an option for people like me.

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.

You mean something like running qbittorrent-nox and connecting to it using web ui?