Taiwan Island only has a few sea ports and few airports. Block them off and destroy the runways (bomb daily after) will cut off all supplies to the island.
The amount of supplies a military force needs i.e. fuel and food are small compared to the needs of the general civilian population. They will absolutely just let the civilians starve to death to reserve supplies for the troops.
There's also the Fallujah scenario where the conventional armed forces surrendered years ago but militia or extremist formations seize a town/city and PLA troops are forced to do a large scale mechanized urban assault. The amount of troops you need to surround even small settlements and prevent leakers is an OOM higher than a storming action.
Furthermore, what do about American garrisons in the Philippines and Japan? The PLA isn't starving out the garrisons in Yokosuka or Subic in any appreciable time frame. The idea of just encircling and starving out troops is a very optimistic and appealing idea but the idea that the PLA will not have to engage in urban storming actions in the future is pretty laughable.
EDIT: in general I find the idea of encircling or bombing out Japan or Taiwan to be pointless. The core issue is expanding the Chinese A2/AD net further into the Pacific to blunt US standoff naval and aerial strikes. Taking Taiwan fast to deploy SAM batteries, land based aviation, and naval resupply points is critical to forcing US Global Strike Command to expend munitions blowing up Taipei instead of Dalian.