Crone
Your router would be the best way, but I don't know much of anything about how specific you could be. If you could set it where if Plex is using it, it gets 100% of whatever it needs and tells your torrent program to fuck off, that would be the perfect solution. Not sure if that is possible, or if it is, if your router supports it. Much less how to do any of it.
Barring that, all you really need is bonus points and seeding time for IPT. I forget what it is but they actually introduced a seed-time goal as well as just 1:1. I want to say it is 5 or 6 weeks. So technically if you just limit your uploads to the slowest you can get during "Plex viewing hours" you'll still get tracker updates showing you are seeding and eventually they will hit the time needed, regardless of upload amount. You'll also be gaining bonus points as you go in case you just want to nuke particular ones to save space or whatever. Overnight you could remove the throttle and let it try to seed as much as possible too.
I don't remember what torrent client you are using, but I know rTorrent let's you assign a default "channel" to all new torrents too. The lowest pre-configured one is "16KiB up" whatever that equates to. So if you chose you could just set everything to that automatically and it should act as a sort of "keep alive" function. Or, you can do things like assign ratio groups, like "1:1" where as soon as it hits even ratio it automatically sets the upload to that 16KiB. That way every new torrent will try to seed as much as it can right away, but once you hit 1:1 it drops to minimum and can't eat up your bandwidth any more. In other words, that one really hot torrent isn't consuming all the bandwidth to get to 20:1 ratio while all your other ones struggle to even get 1:1. Of course, then you might be losing out on upload gigs too, since you just limited your best torrent.
Lots of different ways to configure things and decisions to make, although if you use any of these you aren't going to be able to just do a global limit/unlimit like I described in the second paragraph above. Still, if you aren't willing or able to remember to change it every night/morning (or configure it to automatically do it somehow), setting some of the automatic ones could go a long way to saving your bandwidth. I'm fairly certain other torrent clients would allow the same thing if you aren't using rTorrent.
Not sure what else to suggest, aside from buying faster internet of course, but that might not even be possible in your area for all I know. Whatever you do, don't fucking donate to IPT. You can earn enough bonus points to stay afloat if nothing else, and that guy is a huge fucking faggot and already makes a shitload of money from his sites.