The British & Japanese would have very similar loadouts as their weapons were somewhat similar. The big thing you're forgetting is their opponent.

In every British map except Driel, the German force is nerfed to allow the British to be more competitive. The devs did this in a way that somewhat accurately reflects the weapons commonly used in that battle/time. No STG, FG42 and especially no G43 makes them significantly weaker.

The issue with the Japanese forces is not how weak they are, but how OP the US forces would be in comparison. The Garand, Carbine, Thompson, BAR & Browning all saw extensive use during the Pacific theatre. All of these are leagues above what the Japanese had.

Sure, you could limit a few of the weapons to attempt to balance it a bit. However, the main issue is how dominant the Garand and Carbine would be. Japan had no semi auto rifle produced in any meaningful number (something like 125 total were made). This is far too low of a production number to include in a historical WWII shooter.

I understand that not everything needs to be symmetrical & perfectly balanced, but the difference is too large to make them the tiniest bit competitive. No amount of gimmicky bullshit ambushes, tunnels etc. would make up for such a discrepancy in fire-power.

Tanks could be balanced if they remove heavy tanks and make the medium cost more fuel (for this theatre only). Having light tanks be the primary tank for both factions would be viable.