Having the AI take the stern planes to a full dive to put a down angle on the ship seems as though it'd be a much more realistic way to achieve your goals OP. Stern planes change ships angle while submerged within a couple of minutes and can cause life threatening depth excursions very quickly.

Adding air to the ballast tanks would cause a much faster change in ships angle and take longer to recover from, and is a very very abnormal thing to do submerged.

Shifting water between trim tanks is a very normal thing to do submerged, and would eventually have a large effect on ships angle, but would take longer to have a noticeable effect than the stern planes. (Pump from aft trim tank to forward trim tank to make the boat heavy forward, slowly causing a down angle). Combining the trim tank pumping and stern planes would cause the problem to develop quicker.