Re: Did you use the "invasions"? How you doe that?
The mod is constrained by the engine. Simply put, it cannot support 30,000 units battles. What settings do you have for the armies? There is a huge setting but you will have to have a powerful graphics card to support it.
You need to plan your strategy and tactics accordingly, so if you want to dominate a region, say as Rome (the plan would vary for each faction but the basics are the same), You have your main legion to fight, a second full stack with replacement units to keep your main stack replenished, you could then have a half stack following on with garrison troops and another full siege stack with the artillery. In effect you have your full army.
That said, in some cases you will annihilate the enemy with very few losses on your own side, but you may come up against an elite stack that even your best army struggles to overcome, you may even lose. The mod does the best it can with the old RTW engine, bear in mind it's what 10 years old? So you will have to send stack after stack at times. You may also want to try 1-turn which makes it much slower to build a stack so you will be far more careful in deploying your army, but you won't suffer so much from AI stack spamming either.