@all-encompassing-goose
A few good ideas (separate):
Have truck pieces and supply pieces. Each costs 1 IPC. For every 3-5 ground units (haven’t decided) (including AAA) in a territory there has to be a supply piece there every turn which is consumed. Territories with less than 3-5 units do not need supplies, but they need to have an unbroken connection to a supply hub (the supply hubs are what you designated)
Truck pieces move supply pieces, can move two territories a turn, cannot attack, cannot defend (if found in an otherwise empty territory it’s destroyed automatically like AAA) but can be taken as casualties if there are other units with a combat value.
If units haven’t been supplied for a turn, they get attrition like you showed.
At sea, two supply pieces can take up one transport slot (can replace the infantry) and have to be resupplied every turn.
Because of the fact it’s only required for bigger groups of units, most Pacific islands won’t need resupply but will slow down the Allies in France.
Supply units can be unloaded from transports. Only one can be unloaded or loaded if wanted.
If a territory only has supply units (or with other units that don’t have a combat value) they are automatically destroyed. Supply units cannot be taken as casualties.
Groups of units that number less than 3-5 overseas will be considered supplied if they can trace a path through friendly sea zones to a supply hub (those sea zones can contain friendly warships or no warships, but must not contain enemy warships, transports don’t count).
We can add a Russian Winter effect where on round 3 all Axis units in original Soviet territories need two supply units to sustain the same number of troops or else they get attrition.
The massive problem with this rule is that it necessitates tons of new pieces.