An important consideration to make;
What will be the planetary impact of ground combat weapon systems? Orbital bombardment causes a lot of collateral damage in civilian casualties, destroyed facilities, dust and radiation. The effects of a protracted, high intensity campaign should probably not be underestimated. Scientists have actually been able to trace major plagues and the like in the glacial record because the loss of population affected the way and extent humanity utilized the ground, as well as major eruption events with volcanoes.
While the damage at a low TN tech level should probably not be too bad, this can be very different at the higher tech levels.
Of course, one of the things that may make you want to create ground combat units would be the limited damage you could do to a planet in comparison to a thorough orbital bombardment to destroy enemy units.
EDIT: Please don't strip all artillery, anti air and anti tank capacity from infantry units. Aside from flavour, while modern day man portable weapons are not nearly as effective as vehicle mounted ones, they are still a potent weapon in the right circumstances.
My plan is to leave orbital area bombardment by missiles vs ground units as it is now. For precision, energy-based orbital fire support, the ground units will require some equivalent of a forward air controller to direct fire. I may also do the same for bombardment units (they will be assigned to a forward controller). Essentially, each unit with bombardment capability not in a front-line position will be assigned to a front-line unit that contains a specialised fire-direction unit.
At the moment, I am leaning toward mixed unit types in formations rather than having a single unit type. Units will be designed separately to formations, much as you create components for ships. So you could design a battalion that is mainly infantry based, but with some integral bombardment, anti-tank capability and fire-direction capabilities.
I have changed my original plans so there are now five basic unit types: Infantry, Static, Vehicle, Super-Heavy Vehicle and Aircraft. You can develop armour for each type separately except static, although that can be fortified (armour will affect chance of penetration while fortification will affect chance to hit). Some types can have better armour than others.
Weapons are developed separately and can be assigned to different types of units, although there are some restrictions. For example, an infantry unit can only have light or medium bombardment and anti-vehicle weapons, while super-heavy anti-vehicle weapons (and probably surface to orbit weapons) can only be mounted in static units or super-heavy vehicle units. Due to the base type HP and potential armour, the static weapons would be much easier to destroy but also cheaper.
Weapons have three characteristics:
1) Armour-piercing: Affects the chance to penetrate the armour of the target.
2) Damage: Affects the chance of destroying the target
3) Shots: Amount of fire in one combat round. This is high for machine-gun equivalents to simulate rate of fire and for bombardment weapons to simulate the chance to damage multiple targets.
Infantry have 1 HP, Static 2 HP, Aircraft 3 HP, Vehicle 6 HP and Super-heavy Vehicle 15 HP (might be more for this one). HP (and armour) will be modified by the armour tech of the race in question.
Some sample weapons (as they currently stand)
Personal Infantry Weapon: AP 0 DAM 1 SHOTS 1
Crew-served automatic anti-personnel weapon. AP 1 DAM 1 SHOTS 6
Crew-served heavy automatic anti-personnel weapon. AP 2 DAM 1 SHOTS 6
Light anti vehicle weapon: AP 2 DAM 6 SHOT 1
Medium anti vehicle weapon: AP 4 DAM 6 SHOT 1
Heavy anti vehicle weapon: AP 6 DAM 6 SHOT 1
Super-heavy anti vehicle weapon: AP 9 DAM 9 SHOT 1
Light bombardment Weapon AP 0, DAM 1, SHOTS 3
Medium bombardment Weapon AP 1, DAM 3, SHOTS 3
Heavy bombardment Weapon AP 2, DAM 6, SHOTS 3
Super-heavy bombardment Weapon AP 3, DAM 9, SHOTS 3
Light Anti-Air Weapons: AP 1 DAM 2, SHOT 1
Medium Anti-Air Weapons: AP 2, DAM 3 SHOT 1
(Haven't done air-to-ground yet)
Base AP and Damage rating will be modified by the weapon capability of the race designing the unit. The highest level tech is determined from any of Laser Focal Size, Railgun Type, Meson Focal Size, Particle Beam Strength or Cannonade Calibre. That TL is then matched to the equivalent Armour tech level and the Armour Strength is used as the weapon strength. For example, a race with Laser Focal Size 15cm has the third level of weapon tech. So the weapon multiplier is 8, because that is the value of the third level of armour tech. Using this method allows all races with similar levels of weapon tech to be equivalent in ground combat, removes any complications around converting different weapon types to ground combat and ensures similar levels of weapon and armour tech are equivalent.
Some weapons have other characteristics, such as bombardment may engage from a support position, while anti-air can engage aircraft. I will add some minimal ability for some other weapons to attempt aircraft interception at low chance to hit.
Armour piercing will be based on a formula that takes (AP / Armour) ^ 2 as the basis for the percentage chance to penetrate. If armour is penetrated, the chance to kill a unit will be based on Damage / HP, in the same way a ship component.
The major factors I need to decide are base unit and weapon sizes plus the base chance to hit and how much that is affected by movement (i.e. vehicle vs static) and by fortifications. Also I think I am going to scrap the concept of the Advance position and instead have a chance that formations in support or rear echelon positions will be attacked based on the recent balance of damage between the two sides. Less complex but similar outcome.
More updates as I make progress