Just keep any torrent you download seeding after you finished the download and you will easilly get a seed/download ratio of 2 or more.

Even without static port-forwarding, the NAT translation done by the Mullvad router will automatically keep track of external machines to which your own machine has connected to recently, in order to forward to it any connections back, and since the torrent protocol pretty much connects to the whole swarm during the download stage (even if it doesn’t download from most of them, it still connects to each swarm participant to check which parts they have), which means that after your download stage for a torrent is over, for a while (hours, in my experience) if any of those machines tries to connect back the connection gets properly forwarded by the Mullvad router to your machine because it still recognizes them as associated with your host and forwards the connection correctly.

What won’t work without static port-forwarding is starting seeding from scratch, resuming seeding after you stopped it for a while (a day or more) and very small size torrents (because the swarm changes very fast when the download size is very small and is quickly done, and the new machines in the swarm which your own machine did not connect to during your own download stage, won’t be associated in the Mullvad router with your machine so it can’t do automatic routing of their connection attempts).

The point being that it’s perfectly fine to torrent with a VPN without port-forwarding and you can do it without being a leecher as long as you’re not just downloading tiny torrents and you make sure to leave the torrents to seed for a while after the download stage is over. What you can forget about is seeding from scratch or to remote machines you did not download any data from, which is a problem if you’re trying to get a good ratio for private trackers since you can’t just fire up a torrent for seeding alone and all uploading can only happen following your downloading of that same torrent.