This maybe more entertaining but it also delays the historical aspects and success of blitzkrieg assaults that come at later historical times; so I would disagree against tougher garrisons unless they are in fully Supply. [see Comments below...]
And how many blietzkrig campaigns were a success? 2-2,5. And the main reason for this was technical and numerical superiority (Poland) and low moralle (France and Low countries).
Don't know about the rest of the players but don't want to see any more of these useless garrisons that were less than bump of the road. Taking any garrisoned city was not challenge at all. In later stages of the game, garrisons were something that can be easily brushed aside, without any planning or support.
Moralle and maybe DAR shoul'd have bigger impact on garrison efficiency.
* AAR. After getting a large Civilian Cargo ship into the immediate hex, for @ two turns, Infantry in the assault role get their full green (supplied) color, but then that goes away almost immediately.
And, defending Garrisons remain out of supply status, yet continue to attack and decimate attacking Infantry.
Are you sure it is out of supply??
* HISTORICAL. In fact, the US Army and Marines did land heavy artillery (105 & 155-mm) on secondary atolls that were not defended and then used them to bombard primary objective islands that were under assault. It worked well.
But, in SRGW you cannot land this heavy artillery to support the assaulting Infantry.
In WW1? Wasn't the Aussies and IJN tasked with securing German Pacific colonies??