Ah, you should have asked for an invite code before you started, then you would have gotten a free Diana and Texas.
Anyway,
1. -
2. I played Furutaka and Aoba long ago. When I was playing Furutaka, it was still worse than it is now, so my views may not be 100% accurate. But I liked the Aoba a lot more. To me, it was the excellent sweet spott between the Furutaka and the Myoko (I didn't really like the Furutaka and Myoko that much). I had a lot of excellent games in the Aoba and missed her really badly when i got my Myoko. About the matchmaking: Tier 5 has the worst matchmaking of any tier (tier 6 also has pretty bad matchmaking though, but it's less bad). Just keep in mind that in the current BB meta, playing a cruiser at higher tiers will automatically put endless lock-ons on your ship, so if you would go for it, keep your speed up and hug the shadows.
3. Umikaze is OP at its tier, so don't think all jap DDs will be as good. They used to be the best branch up to tier 7, but patch after patch, they have been made worse and worse, and the next captain skills patch seems to make them even worse. At the moment, I can't strongly recommend any DD, but if you'd want to play DDs, then I'd so go for the American ones.
4. Some patches ago, carriers were overpowered as hell, then the nerf hit. At higher tiers, jap carriers are the best, but i personally don't enjoy playing them. I liked playing them when they were OP, just because playing in godmode was kinda fun. But now, i get more frustrated than I get joy out of it. Also keep in mind carrier captains are crucial in a game. When everything goes smooth, no-one notices it, when it goes bad and you didn't fully do your job, or people are getting killed by enemy planes and you're on the composition without fighters, people will report you for playing poorly.
Suggestion:
Play the German BB line. Best ships I've ever played.