I think many players have problem, when they are playing evil factions, to have good relations with Sauron. Especially when playing WH/WH relations go down really fast. Building temples of Melkor is not enough and relations had gone down before another invasion is on.
My suggestion is that if those little missions like capture x settlement would come from Sauron him self and not from council. This way doing missions will keep good relations with Dark lord.
Problem is that I'm not so good modder and i don't know how to do this, but if somebody will make it, lots thanks and +rep to him