@DallinD
Hey Dallin, welcome!
Assuming Global 1940 and setting aside some of the restrictions the US and Soviets have to making Declarations of War (DoW), I agree with you on the principle of the Axis not waiting around to attack. A well planned J1 attack (less attack on US) is devastating and renders UK / ANZAC decision to declare war a moot point. Furthermore, an immediayr attack by Japan is not only encouraged by the initial unit dispersion but is also historically accurate, as Japan was not raising fears by sitting idle and producing more units. So long as the J1 eliminates the UK and ANZAC wartime National Objective (NO) bonuses before they can collect them, ot makes sense not to wait around.
On the Europe side, Germany taking a turn or two to attack the Soviets makes sense for the similar reasons regarding unit dispersion and historical activity.