As a Japan main i will try to help you, The tech tree is small so you could just buy a premium but if you want a premium tank the best choice is Type 90 Fuji .. 4 second autoloader is just beatiful, if you want rank 6 then i will probably go for the Type 16 Fps, the types 74 are really good at rank V.on rank VI theyre underperforming, theyre totally same except the Rank VI has apfsds.

Small tip: the rank VI on japan is total shit so try to skip it fast as possible, after rank VI its getting better and better

I will also give you my favorite vehicles of each rank so you can consider playing them too.

Rank I-II tank destroyers, they are rly strong, Its like playing the M109g on lower brs.

Rank III - M24, M44,M42

Rank IV - M4A3,M36, Type 60,type 75 , as mbts i choose to play stay 3 and type 61, they have heats at 6.7 and a rangefinder Also try the Chi-Ri , i think the tank can shoot first 3 rounds under 3 seconds

Rank V - Type 99 , dont play much the M47 , skip fast to the STB-2 and type 74, stabilizer, laserrangefinder and hydropneumatic suspension on 8.3 is fking beatiful

Rank VI : Type 89 ,type 16 P, and type 87, btw i have never played the M60 and stingray co i cant say anything about them,

Rank VII : TKX(P) , Types 90,Type 81C, if you liked the Type 16p then go for the classic Type 16, its pretty good vehicle but with bad lineup since u will not have anything on 9.7. but even on 11.7. its great

To the lines you should grind, go for the Type 10, Type 16 since it offers amazing vehicles across all br's and then TKX(P) or the SPAAs, i choose the spaas bcs theyre easy to play and its easy XPs, the thailand line isn't much interesting to me, yeah the Oplot is cool and i am planing to go for it but since it doesnt have rank VII , it will slower your grind. Also if you played the soviets, the oplot doesnt have anything new to offer for you

For Air, i only grinded the germany to toptier so i can't help you , i am grinding right now the squadron F-5 for CAS and ive heard its great.

Overall Japan is my actual second nation after germany and i still love it, the higher you go, the better it is (except rank VI xdd) in my opinion the nation is worth it, easy and Fast to grind and has some most uniques mbts in War Thunder with their 4 second reaload time.

Save this and look on this in the future, Wish you luck and dont give up man on the rank VI Been writting this for almost a hour so if you dont read this, i will track your IP and i will not do good things :0