In qbittorrent, you have to select the torrent file and pause the individual torrent. this will stop downloading or seeding from occurring on the file. I don't know of any way to seed a partial torrent and have it counted as complete for the tracker. This is true with any torrent software, not just qbittorrent. I created a tag for files in qbittorrent that I only plan to partially download, so that I can easily find them and pause them when complete.

Transmission works well with macOS