Searching a simple combat mechanic that allow a lot of unit variety in low quantities

Easy question. But a tough one to answer since the possibilities are almost infinite.

I think that the modifier that you have mentioned, is the best way to go. After all, I sense that you want to keep a
1 unit, 1 piece, 1 die, 1 hit, policy.
It is only a matter of how to apply this in the best way.

Best way in my eyes would be to keep the hit chance as it is. Firebats have 2. Yet applying a -1 modifier on the roll, against a certain hit. Important here is that this modifier happens by accident after rolling. Thus you target a biological unit at the right time with the right unit. And the modifier goes -1 for a certain hit. Rolling 3 means a miss, yet the die awaits its turn in case the 3 becomes a 2 = hit.

For as soaking up damage/tracking health. This forum is overloaded with suggestions already. Just search for "health" related items.

I don't know what you have in mind exactly. But could you describe the rest of the game in more detail?
A combat mechanic often depends on more than just the combat.

Maybe use a POOL of dice which is the MAXIMUM amount of each TYPE of units you can control per "party". Then you can have a tech tree with upgrades than can alter this.

Like say there are THREE (3) TYPES of units:

1. Melee: ground attack units
2. Ranged: ground support units
3. Flying: air support units

You start with something like:

3 Melee, 2 Ranged and 1 Flying with a Maximum of 3 units per "party".

You could have Tech tree advancements such a +1 Melee units (so 4) or +1 party units (so 4 units per "party"), etc.

Then you can use an RPS to control what dice causes a HIT based on the COLOR of the dice.

1. Melee (RED) is beaten by Flying (BLUE)
2. Ranged (YELLOW) is beaten by Melee (RED)
3. Flying (BLUE) is beaten by Ranged (YELLOW)

So Melee could be RED dice, Ranged could be YELLOW and Flying could be BLUE.

You can have other tech tree attributes which control bonuses of the each dice.

How it goes is pretty simple, choose your dice (according to your party: declared attackers) and then roll them. Your opponent does the same... Then you match dice according to the VALUES and COLORS.

So the attacker lays out his dice in sequence. The defender then tries to counter the "attacks". If I rolled a 6 RED, to defend I would need to have a 6 BLUE or a 6 RED. The defender declares what units he uses as his defense... Otherwise the 6 RED would inflict "X" damage based on the VALUE difference: let's say 6 RED is countered by 4 RED = 2 Melee Damage.

And you could of course have a tech tree for additional bonus damage for each type of attack... Like +1 Melee vs Melee attack. So instead of 2 Melee Damage, it would be 3 Melee Damage.

Similarly you could also have units with attack damages (similar to the tech tree but used by units...). Like Firebat = +2 Range Damage, +1 Range vs Range Damage, and a roll difference of 2 = 2 + 1 + 2 = 5 damage.

Anyhow this is how I would see a "Simple" Attack system with Tech tree advancements which can affect different aspects of the dice rolling.

Cheers.

Update: You could also use another "type" of attack "unit vs. unit" or "unit vs. GROUP". So if my damage is 5 Melee and I have the Tech of "unit vs. group" for Unit Type "A", then I can kill 1 Range unit using 3 Damage and maybe damage a 2nd Range unit for 2 Damage (as per my Firebat example).

Update #2: You could have multiple units of each kind for each TYPE of unit. And notice this system allows BOTH players to ATTACK simultaneously using different units based on the dice selection.

Update #3: You may also either want a unit attribute that is "BEATS". Like if the opponent attacks with a Melee of 6, your Flying unit has the BEATS property, it can counter any Melee unit with whatever dice power you want... so a 6 Melee is beaten by a 2 Flying, for example.

Update #4: You can initially have 0 Range and 0 Flying. And would have to upgrade your Tech tree to allow you to "build" these units and in specific quantities... Just like in the real game.

I've designed a handful of games with a combat mechanic, and when I include simple combat, usually I allow each unit to roll a d6, with a hit on a roll of 5+. When there's some sort of advantage, I let a unit roll a d8 instead. My playtesters really like the idea of rolling a bigger dice with more sides, even though it's mathematically the same as your firebat example (adding a +1 to hit on a d6 takes it from a 1/3 to 1/2 chance to hit).

I'll also allow a unit, when it rolls a natural 8, to get a special benefit, such as rolling again or allowing the attacker to choose who dies rather than the defender (The second one has also been very popular among my playtest groups, since it breaks the Axis & Allies norm).

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Another possibility is to allow each player to roll the dice for present units (perhaps bigger dice for more powerful units or other advantages), add them together, and the total equals the amount of "armor" or "health" that an opponent must remove from their force as a result of the conflict. This could be made more interesting by not including health loss tokens to put on units - if you dealt one additional damage beyond what they've removed so far, they have to remove another whole unit, or choose a different set of casualties.

Maybe combine this with an initiative number (I would have 3 layers at most), and have both players roll simultaneously for each initiative number, then have the opposing side remove casualties. I would put Artillery as a lower number, so it shoots first, while mobile units would hit next, and slow infantry or siege equipment goes last, meaning that some casualties may have already been taken out of the slower forces before they even get to fire. Of course, the player who owns the units would choose what units would die, but it would make for a very interesting choice - do I sacrifice my artillery units, which don't hit as hard, and have already fired, but consistently fire first; or do I sacrifice my harder-hitting tanks before they get to fire?

You can play with initiative order in that sort of system very easily - maybe if a unit is in favorable terrain, or has been upgraded, it gets to fire on a higher initiative than before? Of course, upgrades could also cause the unit to hit harder too.

(As I thought about this last idea, I felt like it should be done with dice numbered 1-3 instead of 1-6, to reduce randomness of the rolls a bit, but other dice could roll 1-4, 1-6, etc., depending on the power of the unit in question.

Hope those ideas helped!

That is a rather unusual mechanic.

It is hard to grasp the "logic" to it. Which is needed to get an overal feeling of the usefullness of weapons and defence.

Is it correct to say that an unit with defence of 6: Will always go on rout if the enemy "hits"? Does it also mean that after going on rout, this unit still can be killed by the enemy? Defence is of no use any more when an unit is on rout, right?

Another question(s), just to make sure:
The difference between a target with defence 0 or 1. The weapon is 2-4. It makes no difference right?
What about 1-4. Does it mean that against 0 defence, rolling a 1 means a kill. While against 1 defence, this is a rout. Correct?

Maybe, just writing down all possibilities is key here? I don't know yet. Something tells me that bringing an enemy on rout means a 50% weigh factor. But since it is depending on all the possible defence factors, things are indeed complicated.

Are you using 0 to 6 defence?

Some games have a piercing damage that is always dealt to an enemy. Perhaps you can add something similar?

When 1 unit goes on rout. You roll an extra die. If rolling a hit with this one, the unit is still killed. This could be done with some sort of event die that is rolled with each total attack.

1 side is a certain kill on a rout unit of choice. If you like this idea, you have some more sides to decide on.

Just 1 die extra that is special.
Of course, you could add piercing cards to the deck instead for those situations. I did something similar for dealing with draws.