[Suicide Junkie]

It seems there isn't much choice for troop weaponry...

I'd ditch those ground cannons, and toss in some along the lines of the following:
1) Small, cheap and weak(per kt) - Regular infantry type stuff.
2) Small, expensive, moderate damage - Heavy infantry weapons. The cost should make it so you build these significantly slower than light infantry weapons, but they pack more punch into the dropship, so they're worth while for offense.

3) Medium, cheap, moderate - Regular tank secondary weapons.
4) Medium, expensive, strong - Better tank secondary weapons, but you can't get as many built. Good for offense where size matters.

5) Large, cheap, strong - Regular tank main gun.
6) Large, expensive, ultimate - Better main gun,
These should be big enough that only 1 fits on most troops. Heavy troops might get two, and if you strip off all the armor you could fit three on an artillery design.

If you load the artillery onto the transport Last, they should be protected from most of the damage by the units that went first. They'll get destroyed very quickly if the front lines fall, and the artillery takes direct fire, since they have no armor.