I definitely recommend 1-star difficulty Ran Route for second playthroughs though, regardless of how many points you got in your first playthrough.
I disagree with this bit. It may be the quickest game if you did not get many points in the First Game. Because Ran Route is really easy pickings. But if you picked up quite a few, then I would probably go Kenshin or Isoroku route, just to get them out of the way. Ran route and Kill the Monkey route are best suited for farming points.
The difficulty to a large extent determines how many points you end up with (basic = 60 points to start with, 1 star = 120, 2 star is 150, and 3 star = 200). Since Ran's route is fairly easy, you could easily up the difficulty on that one.
So the First game is always going to be the hardest, unless you are a masochist and play on 5 star difficulty without picking any of the bonuses. The key is to pick up the bonuses you can get, particularly the Mouri bonus. All 4 of those are extremely useful.
Think I went Basic Game, Isoroku, Kenshin and then Ran in that order, increasing difficulty by 1 star each time), on the fan translated version, and if they have not changed the mechanics, that is probably the easiest way to unlock everything. That Ran route netted me 295 points, and with the 20 bonus points for finishing each route I have 315 - exactly enough to pick all bonuses for a fifth playthrough.
The hardest bit may be capturing the girls from Masamune (which can be done in the first playthrough) and the Takeda generals (second playthrough onwards). You really don't want to have to do that on a high difficulty, due to insane troop sizes.