Key phrase: "if you want." I didn't say you should, or that the Nagato is not fun, but the fact remains that the Fuso has the same number of crew slots and more displacement to spare, which, while originally irrelevant, is now rather valuable in a world where subs exist. Additionally, the lower range is made up for in the 50% greater firepower and smaller size. Add to that the bonus for hitting higher ships, and the Fuso is better for leveling.
With the B65 VS the Fuso, the extra support slot is negated if you don't get as much experience per battle. If you get 15% more experience on average in the Fuso, it's still going to level your crew faster. If you get 25% more, the B65 becomes a pier ornament. Again, it all comes down to skill and play style, but most at-level players will see more benefit in the Fuso.
As for event BBs, I actually have them all (IJN is the only nation where I have every event ship, with KM second at 1 short).
Mikasa is a fun ship but extremely hard to deal any significant damage in - I think it may actually have the lowest damage per unit of time of any BB in the game. But it is fun with a high level crew.
Haruna is nothing special unless your gunners are at Nagato level (can fit 16.1" Ls with 2 binds), in which case it becomes a Nagato-light with better experience due to the lower level. At-level, it grants no appreciable benefit whatsoever, and the only benefit it provides over Fuso/Ise is the slightly lower level requirement.
Mutsu is just a Nagato with 1 extra support slot that almost block shots. Nothing more. Nothing less.
Hyuga is a beast, and arguably one of the top 5 EBBs (with Giulio Cesare 1914, Le Havre, Shinano, and Maryland IMHO), which is a beast even after getting nerfed and even at level. The only thing you won't like is the awkward gun placement. But it's basically a Nagato that hits like a Fuso. That's all you need.
Musashi, like all in its tier, plays the exact same as the Yamato, with the only real benefit being the extra support slot.
Shinano, as I already said, is one of the most powerful (and fun) EBBs to play. The lower speed and crew compared to the Musashi might put you off a bit, but you'll probably be coming back to the Shinano while the Musashi is collecting dust.
Mikawa is pointless. Crappy AA for IJN and the best guns for it are the Shinano's! The extra support slot is not worth the money or medals, and considering fun and leveling value, I just don't think it's worth it. Besides, at that level you just need an event and World at War rooms to blow through the last bit of the grind, regardless of ship.