It's kind of funny that a starving city can just crank out a couple tanks to mitigate starvation so I have two solutions.


The simple way is you just have it so once a city produces a unit it keeps the food requirement for that unit until its disbanded.


 You could also do it so each territory has an equal amount of food taken from it so it doesn't burden your most productive city. Units could require a direct pathway from them to your city in order to receive food supply lines. If this is blocked by the enemy the units will start gaining a health penalty as the turns go on. You could have it so this is mitigated by pillaging farmers quarters. This would make conquering far off territories more dangerous and incentivize plundering.


The game really only rewards cultures that can take things easy and wipe out their neighbors so by introducing a new challenge for those seeking world domination it could make it so everyone has to play at least a bit nicer. Afterall you don't your units colonizing a new continent to die of hunger because your supply lines are blocked by a coalition of your angry neighbors. This could make having open borders even more important.