Honestly, I see absolutely no harm in allowing ANZAC to declare war on Japan prematurely.
Why? Well, there are many points to why it may not be an entirely great idea for ANZAC to do this since Japan does massively out-weigh them in military might and, even if they are distracted with Chinese Territories, there is very little for the Japanese fleet (surface warships primarily) to do. Doubly true in Alpha +2 due to the severe restrictions on Russia initiating an attack on Japan, the loss of a reason for Russia to initiate an attack (to collect Neutrals in the Middle East for cash) and the reinforcements to what we used to call Manchuria (now Manchuria and Korea.)
However, let’s pretend that Japan declared war on USA before Round 4. (for instance, Round 2 maybe.) Maybe England does not want to be at war with Japan yet but ANZAC could really use some action?
Now, I know that Japan declaring war on England/ANZAC brings America into the war, but I do not believe that declaring war on America brings UK/ANZAC into war. (And I am thinking of strategy that makes more sense in attacking America early and ignoring China.)
Since we’re really only talking about ANZAC declaring unilaterally, and not giving Japan the option to only declare against ANZAC, I do not see a downside here.