Building a combat system for large battles

[Hirax]

I'm trying to devise a combat system for a risk/div hybrid that's D&D themed. First, there are four types of forces: army, navy, air force, and special forces. Then, you can make as many different types of subunits as you'd like. For instance, one air force subunit might consist of griffon riders, another raptorans, and a third might be airships. The reason for allowing multiple types of subunits is that they all have different costs to build, and different upkeep costs. Each subunit will have the following relevant combat stats:

Power: Default is 1.2 for special forces, 1 for others.
Combat skill modifier: this is a player score that all units use by default, though through research, training, etc. subunits can raise their modifier. Think of it as an ability score modifier or skill modifier in D&D.
Terrain: skill in various modes of terrain, which are earth, aquatic, and airborne. Defaults are are army having +2 on earth, navy +2 at sea, airborne +2 in the air, and special forces +2 everywhere.
Quantity: number of subunits

Units consist of 5 subunits of the same type (no mixing). Units with less than 5 subunits have their contribution to combat reduced to 80% if it's a unit of 4, 60% if it's a unit of 3, etc. Each unit contributes a score to a battle, and all unit scores are totaled to form a player's total attack value. Here's the formula for a unit's contribution:

((quantity+skill mod+terrain+circumstance bonus)*power*circumstance bonus+1D20)*(quantity/5)
Circumstance bonus occurs twice, once for the addition phase, and again for the multiplication phase. The most frequent circumstance bonus will be a 1.2x multiplier to the defender on their home turf, for instance.

So, let's say Jack (with a skill mod of 4) sends the following to attacks Jill's sky fortress:
An army unit of 5 human warrior subunits
An airforce unit of 5 raptoran wizard subunits

This would output the following attack score:
Human warriors: ((5+4+0+0)*1*1+1D20)*(5/5)=(9+1d20)*1
Raptoran wizards: ((5+4+2+0)*1*1+1D20)*(5/5)=(11+1d20)*1
For a total of 20+2D20

Jill (with a skill mod of 3) has the following at home to defend:
An army unit of 3 elf warriors (who have an upgraded skill mod of 4 due to their advanced weaponry)
A special forces unit of 3 super wizards
Elf warriors: ((3+4+0+0)*1*1.2+1D20)*(3/5)=5.04+1d20*.6
Super wizards: ((3+3+2+0)*1.2*1.2+1D20)*(3/5)=6.912+1d20*.6
For a total of 11.952*2d20. The non-D20 calculations will be handled in Excel, so math slowing things down won't be a worry.

1. Are there any ways to create a metagame advantage with this system? For instance, I tried to make it so there's no incentive to split up your forces into lots of >5 units, so as to add your skill mod and extra d20 rolls additional times. The penalty to units of >5 achieves this, I think, unless I'm missing something.

2. Are there any suggestions you would add for unit statistics, or ways to come up with losses?

3. Could the formula to calculate a unit's attack power be improved in some way?