Battles

We don't need a full war-game simulation system, if we wanted that we'd play 40k, amirite? So instead of a tactical system, I would propose having each leader favor a style (Aggressive, Defensive, Reactive, etc), and based on that, have a probability of picking certain orders (Charge, Hold, Ambush, Retreat). The combination of orders, along with the strengths of the forces, would be compared to pick a result, determining win or loss.
Then we get to individual battle deeds. We want to know how our little mans performed. I've sketched out my idea for a Tactical Marine battle deeds table. THIS IS ONLY A DEMONSTRATION, and made to be simple enough that you can play around on it with just a D20. After a battle resolution, roll on this chart for each tactical marine. (naturally, the computer would do this and append the result to that brother's history. the goal is to have a long history of heroics for each marine.)

For an anti-tank weapon, add +1 (meltabombs) +2 (melta, plasma gun), +3 (missile launcher), +4 (lascannon) +5 (multi-melta). For anti-infantry, -1 (close combat weapons) -2 (flamer, plasma gun), -3 (heavy bolter), -4 (heavy flamer), -5 (plasma cannon).

Note that 10 and under are skewed towards anti-infantry antics, and 11 and above  for anti-tank, so that the result is more likely, but not certain, to reflect the marine's armament. The two center results cause the marine to die. If these are rolled, the marine can roll a D6 and may reroll on a 4+, and otherwise is killed regardless of armament. There are also injury results, getting heavier towards the center, that will require medical attention.

(In an 'actual' version of this, these results would each lead to a separate table of imaginative ways to die. For that matter, most of these would lead to separate groups of random details, and there would of course be more modifiers. Certain enemies, like Chaos Marines, or certain situations, like hopeless outnumbering, would have modifiers towards the center of the chart.)

The point is that rather than micromanaging each battle, we get casualties, kills, and awards, and can go back to the chapter managing part of Chapter Master.

D20

-5: Brother [name] slew 6D8 [enemy] that day!

-4: Brother [name] slew 6D6 [enemy ] that day!

-3: Brother [name] was surrounded and took down D20+D8 [enemy] before he entered hibernation, sustaining only minor injuries.

-2: Brother [name] blasted 4D8 [enemy] apart, and D6 more fell before his combat knife.

-1: Brother [name] made good on his vow to beat his previous kill streak, surpassing his old record by D8 kills.

0: Brother [name] was first to sight the enemy, and with well-placed shots took down 3D6 aright, earning a commendation for the Marksman's honor.

1: Brother [name] lost his left arm, but subsequently found it. In the meantime, he killed D12+3 [enemy].

2: Brother [name] acquitted himself well, accounting for 3D4 [enemy] and guiding the squad through the haze of battle.

3: Brother [name] was separated from his squad. Hunting alone, he overtook an enemy squad. His ambush caused such panic that 2D6 enemy were killed in the confusion and the rest fled.

4: Brother [name] was immobilized by a blow to his leg servos and was evacuated from the field of battle by chapter serfs, but by then had already claimed the lives of D12 [enemy].

5: Brother [name] later made it known that he did not enjoy the battle. He does not comment upon his kill tally, and it is rumored he is in a foul mood because he claimed no enemy lives.

6: Brother [name] was seen to take position ahead of his squad at a narrow choke point, revealing himself as the enemy drew near. The shock startled his foe, and in an instant he had slain D4. The rest fled.

7: Brother [name] suffered minor injuries, but destroyed D6 foes before returning to a safe position.

8: Brother [name] was at the fore of the line as his squad emerged from cover, daring the blasts of the foe, and was indeed struck, suffering moderate injuries. With the assistance of his companions he was able to fight on, defeating D4 [enemy] with careful shots.

9: Brother [name] charged overboldly ahead of his squad, and was overwhelmed by the volume of fire he drew. He collapsed into hibernation with heavy injuries.

10: Brother [name] knew the odds he faced; he willingly laid down his life, slaying D20 foes in his final minutes.

11: Brother [name] chose to stand and face a [tank or monster] for death or for glory, but was crushed under its mass.

12: Brother [name] climbed onto a [light vehicle, walker, small monster] to better place his krak grenades; unfortunately, he became entangled and was caught in the blast, suffering heavy injuries.

13: Brother [name] brought his weapons to bear against an oncoming [light vehicle, light monster], emerging from cover to get a clear shot. He destroyed the foul construct, but drew much fire, suffering moderate injuries.

14: Brother [name] is recorded as having spent his entire supply of ammunition and explosive in destroying an enemy [medium vehicle or monster]. Unrelatedly, he caught a powerful shot to the chest, resulting in only minor injuries.

15: Brother [name] is was seen to draw fire away from his squad. While the proper kill attribution remains unclear, his squadmates credit his actions with the resulting destruction of D3 [Bike or fast monster].

16: Brother [name]'s right arm glove motivator was critically damaged in close combat,  crippling his ability to fire a weapon. However, tossing grenades with his left, he destroyed an enemy [heavy tank or large monster].

17: Brother [name], in a fearless show of Space Marine heroics, climbed aboard an enemy [large monster or heavy tank], [killed its crew/severed its head], and lived to speak of it.

18: Brother [name] was found in hibernation beneath a chunk of rock. He has no memories of the battle, and his pict-captor was damaged. His ammunition supply, however, was almost entirely expended, and it is probable that the wreckage around his resting place was his doing.

19: Brother [name] destroyed a [light vehicle, walker, small monster] with his fists alone.

20: Brother [name] brought down a lumbering [heavy tank, large monster], and went on to slay D4 enemy and another [light vehicle, small monster].

21: Brother [name] was seen roaring his victory in the smoking remains of an enemy [large vehicle, large monster] when a squadron of [light vehicle, small monster] approached. With consummate skill and assistance from his squad he destroyed each in turn.

22: Brother [name] used his heavy weaponry to annihilate D3 [medium vehicle or monster] before approaching infantry forced him to fall back.

23: Brother [name]'s squad feigned retreat, drawing a formation of [heavy tank/monster] past his concealed position. Stepping from the shadows he destroyed D3 of them before abandoning the effort for a new objective.

24: Brother [name] blasted D6 [randomize class of enemy vehicle/monster] apart that day!

25: Brother [name] blasted 2D4 [randomize class of enemy vehicle/monster] apart that day!
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To see how it works, just pretend you're following one battle-brother over his career. Roll again until he dies, picking up a history of ridiculous heroics.

Now imagine this, computerized, compiling histories for every marine in every battle with even more detail.