Is Rank 10 and up significantly different than before or are there still plenty of plebs who don't bother with angling and like handing out citadels like party favors?
Rank 10 to 5 is pretty tame. Its from rank 5 upward that the skill level becomes significantly higher. Though I would imagine that by now the really good players have already reached rank 1, and that the 'above average, but not incredible' players are still around that area right now.
Either way, the difference between rank 6 and 5 is significant.
Having gone to rank 1 with a Nagato myself, I'd recommend against it
She's just too soft at most angles, and will get regularly penetrated for 4000 damage hits even at 'good angles'. Tirpitz and Atego (the repair really helps) are both acceptable, but I don't know about bucky. What mostly dominates the DD matchups are russian and american destroyers. The gunboats that can beat any IJN destroyer mano o mano, and can invoke pure cancer on every other ship. With that I mean, they can pop smoke, sit still in the middle of it and just constantly pour out a rain of fire on anything in range. No counterplay possible against it in a battleship.
Every matchup I've been in, in which my team had Hatsu's or Fubuki's vs Bensons and Kiev's was lost during matchmaking.
Also, most players at those ranks will generally be pretty good in avoiding long range torpedoes.