My choice would be to do this on the router using QoS management. A lot of routers sold the last couple of years have this feature (check your routers manual).

Simply put you specify that one source of traffic (example: http over port 80) should have priority over other sources (bittorrent), and/or that some devices have priority over others.

Depending on your router the settings can also be set individually by macadress (<- unique device identifier) for example to give your phone priority over traffic from your computer or give your devices priority over guests who happen to connect.

As a bonus this will make your torrents run all the time, but (those few seconds, or during that streaming of youtube) when a device makes http requests the torrent traffic will slow down during the requests and speed up again when other devices are idle.

I know this wasn’t exactly what you where asking, but I suspect this is a better, and more dynamic way to solve it, rather than stopping and starting your bittorrent client (with possible implications of re-scraping, finding and connecting to the peers again etc.)