In-service vehicles and heavily upgraded variants that distinquish them from other trees are conflicting descirptions. Lend lease vehicles in WW2, like the Sherman, were in service by the Russians but they didn’t make massive changes to them. Taking Shermans away from Russia would be against your “in-service vehicle” requirement because they actually used them in combat.
Germany past WW2 uses allied (US/Russian) vehicles because they were occupied by those nations. Pulling those vehicles makes very little sense because it would destroy a large section of the tree that benefits highly from having those vehicles. This is true for Isreal, Italy, China, and Japan. Having vehicles fill in gaps where neccessary is more important than restricting “nation specific vehicles” to the originating country.
Where does it say that Warthunder is a Nation vs Nation game? Arcade modes pit nations against themselves all the time, Naval RB does, Air RB does, and Ground RB does at certain BRs and times of day. The only forced sides are Sim game modes, and they can be quite unbalanced.
Sim isn’t the primary game mode of Warthunder. It is a small niche in the game. They shouldn’t balance the entirety of the rest of the game off of Sim because they aren’t the majority of players. They also have the ability to restrict vehicles in the Sim listing, which could help your issue without hurting everyone else.
Also, doesn’t Sim have an identifying mechanic (in ground at least) tied to rangefinding? If you can’t make a positive ID your crew can do it for you, am I correct?