Also.... seiges:

Look at the map. Is every single settlement swarming with garrison troops? Nearing the beginning of the campaign, almost no settlements have an adequate garrison. That is why blitzing is possible.

If every single city had a huge garrison, blitzing would be more difficult.

Go where the weakest point is. So what if Bran has a huge garrison? So what if Frankfurt or Paris have huge garrisons? Go around them. If the garrison leaves the city, send in your spies and take the city by force, sack it, pillage it, burn it to the ground, and ignore the enemy stacks. They are so passive and you can run away, and when you can't, you can slay them.

Assaulting France, hypothetical:

Paris has a full garrison, and another settlement has a full garrison, others have half garrisons, some have few troops at all. Go for the weakest places first, take what you can with the fewest deaths, and sack the provinces. Now move on to the next biggest threat until you end up seiging the huge garrison province. By then, the enemy's empire is wiped out and they have to sally or starve to death. In the meantime, another stack is wiping out another empire.

Easy as pie. If the enemy doesn't sally, you win by force. If the enemy sallies, you win because you're a better commander.

The only way this gets complicated is if you "turtle".... ugh.... and the enemy has huge stacks everywhere. Well you can do that if you're really patient and you like leaving the enemy avenues of resistance. Personally I just exterminate everything as fast as humanly possible.

If the script were modified to give the enemy more soldiers and money, then it might be more challenging.