Combat/Micro managing issues

Statistics:
Why defence points and health points?
It sounds to me like they could simply be added up and then be compared to the damage.
The only 2 logical answers that comes up in my head are: There are units that have more then 1 bullet to shoot? Units are allowed to team up?
In those cases, the defence points are subtracted multiple times. Or else there is no value in having 2 statistics.
If you want to know how strong your unit really is according to math, please ask me. That system that you are using, plus the things that I have described here, I played a certain combat card game in the same way years ago.

1 combat (my war game, maybe you see something useful):
In my combat, the combat itself costs 1 action point. The defender has to pay 1 too if he/she wants to defend. All units are allowed to only shoot once their bullets. Some units are allowed to fire sooner then other units, which are sub turns. During the battling, each player is allowed to throw in "usable" Event Cards at any time (RTS aspect inside the sub turns). Players are also allowed to "help" their enemy with an Event Card. And if there are more players, they too can meddle in with Event Cards.
Anyway, my units are allowed to:
- team up naturally (big units can die now),
- to hide behind each other (walls have use now, the unit requires minimal range, multiple lines are allowed, it can be done only once each battle),
- and the damage remains (big units eventually die, healing and repairing is useful)

Event Cards:
Some of the cards that I thought of could aid you as well, although I don't know exactly how and when you use your cards precisely. Mine could be used any time unless the card tells the restrictions.
Here is a copy of my cards manual, modified a bit to your tastes:
3 Backdoor
The selected squad may only attack the last line instead of the front line. This card must be played after deciding the lines, but before the dice have started rolling by both players. All units are selected for this.
4 Flank
The selected squad may attack 1 other line of choice instead of the front line. This card must be played after deciding the lines, but before the combat takes place. All units are selected for this.
5 Backdoor Advanced
The selected squad may attack the first and the last line. This means that some units attack the back and some units attack the front line. This card must be played after deciding the lines, but before the dice have started rolling by both players. All units are selected for this.
6 Flank Advanced
The selected squad may attack the first and 1 other line. This means that some units attack 1 other line of choice and some units attack the front line. This card must be played after deciding the lines, but before the combat takes place. All units are selected for this.
7 Surrounded
The selected squad may attack any line, all at the same time. All units are selected for this.
8 Surprise
The selected squad may fire all their projectiles, from all ranges, before the enemy may return fire. Less units may be selected for this.
16 Health Permanent
The selected unit has 1 more amount of armor added to its amount of maximum health for the entire game. This card may be played any time, especially when the unit is about to die by incoming projectiles. Place a Health Permanent on this unit and place the card back under in the deck.
17 Damage Permanent
The selected unit has increased projectile effectiveness for the entire game.
Place a specific Damage Permanent on this unit and place the card back under in the deck. You can only increase 1 Damage type. (I have multiple weapon systems, like mammoth tanks)
18 Projectile Permanent
The selected unit has 1 more multiplier for the entire game. Best used on units with multipliers of 1, which effectively doubles the damage. If an unit has more damage types, then each type increases with 1 extra Projectile. Not to be used on units without damage. Place a Projectile Permanent on this unit and place the card back under in the deck.

Some "strategic" stuff with eXperience Points:
Due to various reasons, I have left out the XP part until the end of the battle, where players are allowed to distribute the total XP amongst the living units. This goes way faster, and all XP can be kept. If it where during battle, players needed to keep trace of which unit gets the XP and once placed, it could probably be removed with that dying unit again.
If one player is evil, he/she would simply attack from another region while the defending player has "tapped" his army by placing the used action point on top of that region.

So your units, need to get extra cards for having armor?

And do you have differences in unit strength? Meaning, that some units are indeed stronger than others when all stats are added up? Which makes it harder to balance.

By the way how I read your action spending. It seems the battle between units is always 1 on 1. Is that true?

Perhaps, you should rethink your combat from scratch. (it helped for me) If you are going to copy RTS in this matter as well. I suggest that you play different RTS games.
Your combat reminds me of Warcraft 2 with only the basic units. Although, you only have 1 type of damage, Warcraft 2 has piercing and basic damage. Perhaps by looking at how battles go in Warcraft2, you might get some new idea's. Starcraft 2 also has some sort of system that looks like your system.

You also could add some sort of first strike or dual strike on your units.

Could you give some examples of battles? Like how it goes now? That could give clearer vision.
Telling what is on the field, what players have in their hands and explaining each step that occurs.
1 where 1 player wins, and 1 where both players end up in a tie (dying or not)

What kind of strategy/tactics are possible now?
And what kind of strategy/tactics would you like to add?

Well, it starts making sense now.
The cards keep in position. You are not doing a new battle over and over.
With the 3 by 5 field. You have some sort of 15 regions where cards could be.
The battle does not really sound fast to me. I don't think that big battles with a lot of actions can't be done within 15 seconds.

Adding that one extra attack power gives that scout an edge over the enemy scout.
There is indeed not much strategy in this. Except using the best of the best.

If you want to have more tactics, perhaps giving each card, 1 strong point and 1 weak point. For example, Sniper has +1 to infantry, but it has to get -1 against something else. Like air?
You could also add +2 to infantry, then -2 or 2 times -1 from other types. As long as you keep balance. Although, you also need to keep watch that one of your cards doesn't get overkill. Unless, the enemy scout has so much armor, that you have no choice but use a sniper.
This way, you place certain units into battle in a later stage of the game. Not needed at first, but certain needed later on. Also ads to strategy.