In the radio Drama Leia specifically calls to use the ion canon on the AT-AT walkers but is told by an officer "They cannot target at this angle"

So that's something they did think about.

The Ion canon would have fired several blasts every time a transport left with fighter escourt because Leia tells them that in the briefing.

Although they had to evacuate 2 transports at a time as they knew the battle was not swinging in their favour.

Apparently many transports were actually destroyed, but we never got to see this.