Good idea, but I don't have any facility for doing that in the routers, or I can't find it anyway.
They are TP-Link MR3420's with Sierra 320U USB Modems plugged in. It seems as though they support DD-WRT, which could possibly be made to do this, but I don't think they would then also support my USB Modems.
I think that if I can work out how the Trigger Level in Routing Groups is created and works, I may be able to make a new Trigger Level that uses Total Traffic for an interface. Unfortunately for me I haven't the faintest idea how the Trigger Level works, so it's going to take a few days of spare moments to find out if I can even do this. Any hints or tips would be appreciated. Meanwhile, manually controlling the connections will continue to work.