Actually there are some reworks needed regarding attrition, supplies and other similar stuff. This would need some reworks in AI. What if we had "supply" bar for every stack? Another bar just like morale but decreasing with time. If it's empty - then it's the time when army will suffer attrition and morale loss. Looting should pump supply bar up and also having fort controlled by us with clear passage. If you for example entered Russia without securing forts on your way... You should be screwed badly. And right now - you suffer attrition, your reinforcements just spawn in your army, your land is like 400 km away from this place. How do they even know where to go? How are they not killed on their way? Forts should be "checkpoints" that we want to have in our country and that our enemy would want to take, because if we cut them off they should not be able to reinforce AT ALL. They should not suffer automatic attrition if they still have some supplies, but pillaging can't last forever for really big stacks. Supply lines should go from our lands (captured in middle of Siberia fort would give supplies, but no supply line, supply line needs to go from our land) OR from ports controlled by our fleet (hey hey hey! fleets would be useful!). Scorched earth would be viable tactics. It should work like looting our own lands (loot bar = 0) and then we should have no supplies (our armies should eat these too if they are too far from fort) but our enemy will not take anything either. And then Otto stack going for Moscow will have a really bad time if scorched earth tactics will be in use. Napoleon knows how it's like. But they could move slowly and take forts one after another, and then supply line would go from their lands or lands of their allies and keep them alive.
Also - army using supplies should cost more. Actually not army, but supplies per se. During peace time armies should be "mothballed". They have full morale and stuff, but minimal number of supplies, like 10% of maximum. It's enough to take rebels for example, but by no means enough for military campaign. Every battle should use some supplies (for example per day * regiments in use, with proper modifiers) so we can have 1-2 battles with rebels without getting attrition and morale loss, but would not last long in enemy territory if supply lines are not good enough. Supplying our armies would just need some money investment when they are in fort ZOC in friendly lands. In game mechanics that should be army maintenance bar. Bar is full = supplies are full, all the time ready for war, cargo carts all always packed and ready for orders etc. But it should not be this punishing to lower it, because it should not affect morale or ability to fight, but only in short term. 20% bar = 20% supplies, they can supress the rebels, they have enough resources for that, but in battle with fully supplied army they may use all their resources during battle and that would create massive morale loss and risk of stackwipe. War should be over when our armies have no supplies and we don't have money to provide them.
Player would manage this mechanics rather easily, question is if AI would manage it too.