So you have five PPPoE clients on ether1 and they all work, i.e. they connect and get addresses (I never tried if that works)? If so, it’s like you have five WAN ports and you need to do the same thing as with any other multi-WAN setup. In magle prerouting, mark new incoming connections based on incoming interface. And then in mangle output, mark routing for marked connections, to use the right WAN.