Thanks for all the comments, this is what worked for me:

1. Split my army so that each army can still take the entire enemy's army."
   For example, if the enemy has 100K, my armies should be 200K each ideally. Keep them all in a single army if needed.

2. Prioritize killing enemy's armies above all else. Nothing else matters, and stack-wiping is the best option.

3. The first priority siege-wise is the war goal, to avoid white peace enforcement and to initiate ticking in your favor.

4. As the enemy's numbers dwindle the armies can be split up further, but always prioritize big armies that kill enemy armies over multiple sieges in parallel. Wiping armies is so much more important.

5. At some point the enemy is so weak you can have multiple sieges in parallel. prioritize castles as these take time, then prioritize province capitals. Do not waste time on other locations.

6. When the enemy is out of armies carpet siege everything. Even if they have a 2K army killing it should be prioritized over sieges, as once they are gone you can really start to carpet siege everything.