you need motivation. that is what you need.
you mentioned that you struggled between 1.5-3 mils nw each age.
try to aim for a 2-3k acres provinces like a 7 mils nw or so at the end of the age. try to aim for a top 50 land/nw kingdom.
or try to aim for a barons kingdom.
this will motivate the new commers. is the only solution i believe.
when they join, you can teach them heaven and earth, if you do not feel the little taste of success and involvement they wont put any more feelings into this game.
motivate them with high goals, involve them intro kingdom strategies (make use of inteligence of every player and even if they are stupid ask them what they feel about this strat or that or this). make them feel important part of the kingdom and of course important part of the utopian community.
when a player feels important part of the utopian community he will never lose interest, on contrary he will involve more to get better himself. sooner or later he will figure out that without team he cant get anything alone, therefore he will involve more to improve the team.
i hope you got the picture:)