Uh ... I think I understand ... what if you simplify it this way:
- Each port/field has a number of cities associated that's related to the size of the field (small's have 2, ports and meds's have 3, larges have 4).
- These cities are layed out along the road network kind of like you've done.
- Each city has a vehicle base in it - so you could theoretically capture the cities from the ground if you wanted to. There should be enough distance from the cities to the associated airbase than taking the airbase by ground alone is not a time-effective tactic. However, city-to-city land battles should be frequent.
- When these cities are captured, then the associated field can be taken.
- AAA cover at fields is doubled since the action is to take place over the cities and only when that's all done is the field capturable - meaning it'll need to be a serious and costly assault. Defenders can only hug ack when the field capture is immenant.
- This obviously means you need fewer airbases since there are 2 to 4 peripheral targets to fight over for that sector. So fights wouldn't necesarily get spread out more, but the overall number of objectives would shrink. The "world" we fly from gets smaller because there are fewer bases.