Does Transmission let you force the use of a specific connection?
For example, qBittorrent lets you choose your VPN as its only allowed connection so that you can't accidentally use your regular network when not connected to the VPN.
Never thought of that. In my use case, Transmission is running in a container on my server, so it only ever has one connection, and VPN and traffic management happens on my router.
in qBittorrent: Tools > Options > Advanced > Network Interface. That dropdown box should list things like your Wi-Fi connection, Ethernet connection, etc., including an option that sounds like your VPN provider. If you run into troubles, do an Internet search for "qbittorrent network interface" for various articles and guides.
In the settings panel, the Advanced section has a setting for network interface, with a drop down menu of your available network devices. Your VPN connection should appear as a separate device in the list. If you choose that, it will only send data through that device, so if you're not connected, it has nowhere to send data to.
The github only has a description of the setting, and doesn't really explain it any more than that.
Yeah, Transmission is pretty nice, there's nothing wrong with it. It's also pretty popular among macOS users, because it looks and feels like a native app.