I'll agree with Glzsqd. The Hellcat requires careful energy management, very careful, and (like your Brew) lacks speed, but is otherwise an enormously capable plane, which will also teach you quite a bit about flap usage. People whine and moan about its limited rear view, but frankly I've never found it to be a problem.
I'll disagree with the others about the Spit. If you've learned to fly the Brew well, you can also fly the Spits well.
- oldman
Yeah the F6F is good, but I think it's little more difficult plane to learn to learn for the "next step" after flying the brew so much. I think the F4U, F6F and NIK almost fly exactly the same. Butttt each have unique preferences..my fav out of all of them is the f4U. The spit is prob a bit easier to fly. And I like what bigrat said about that flap work of the spit. However these flaps are the reason why a lot of people aren't effective in that plane, because deploying them too early can kill your E state and not putting them up at the right time can cause you to be too slow.
The thing is though bagrat, is that much like you said about the excess ammo will create bad habits , so will only being able to use 1 notch of flaps in fights.
Oldman. People " think" it's easy to fly and get kills and land them, but the majority of spit pilots you meet in the game aren't really that great because they don't understand situations and aren't good enough at defensive maneuvers to be effective. The way this plane flies is meant to get down n dirty but you have to know how to get out of the furball zone.
The FM2 is the same as the Brewster. You won't be developing anything new in your skills archive.
The nik or F6F (which is even harder) or the spit 9 would be your best options. Hell I'd say fly them all. You don't have to to only know one plane.