I currently have a hard time remembering my last battle, even between defensive ships, that was not over, or at least decided, after the first phase. This tendency gets worse in the late game. My simple proposal would be to slash conventional weapon damage by 25-50% across the board. This would have the following effects, IMO all of them positive:

1. Buff to short range weapons and tactics: A fight between one side choosing a short/medium range battle card and another choosing a long range begins at long range. Combined with cross-flotilla fights being (mostly?) long range, this makes short range tactics very situational. At least defensive fleets should have the chance to actually get into close range without the cooperation of the enemy.
2. Buff to defensive techs and ship classes: Fleets made up mostly of attackers currently just feel much stronger than fleets made up mostly of defenders. It should be a strategic choice.
3. Buff to underused modules:  squadrons, repair/shield reloading per phase, boarding pods: Squadrons currently effectively deal little damage partially because they take a long time to reach their target. I only tested out boarding pods once, but then my carrier with three large pod modules did about 10% troop damage to a ship before it was blown to bits by the rest of the fleet, so pods do not appear to be exactly OP either. 
4. Conservation of forces: This may be debatable, but I would prefer it if more battles did not end with the compelete destruction of one side. Without the AIs huge bonuses at higher difficulties, I think most empires only field about 1-3 fleets, and it would be nice if wars were not decided in one battle. This is pretty subjective though, feel free to object :)

This is something I've chatted about before on discord and can agree with. It's very glassscannony atm and defensive ships seem to always get blown up in a fight as they get focused down by the overwhelming damage from attacker ships. Either you lose a defensive ship or your whole fleet. You could say the defensive ship did what it's supposed to do, but this happens when you got a fairly one sided battle as well.

There's a lot of good points mentioned here, and personally, I don't have much to say. I'm not that knowledgable about the combat and numbers on how to balance it, but I can atleast give an opinion I feel the fights do end too quickly even when it's fairly even on both sides. It never reaches short range unless there's something ridiculous that happens like the flotilla lanes that are left moves in such a way they can't fire at eachother.

One idea I had was to spread the damage from all ships, which might fix how defensive ships are just nuked down by the attacker ones. Attackers will still focus defensive and vice versa, but some portion of that damage is spread across all ships in the flottila. That may even make fights last longer with more ships getting hit and some damage is spread instead of all being focused.

Foraven wrote:

I don't like the idea of having ships used solely as meat shield for the rest of the fleet. I especially don't like that they are the ships we can fit troop slots/bombardment modules on early game and we can't change their guardian attribute so the rest of the fleet can protect them instead. 

You can fit modules that encourage or discourage targeting. So you could have other ships increase their weight for being targetted and move attention away from the support ships.

One thing I DO agree with is having defense modules give an even higher defese bonus on support ships to make those ships last longer. That would certainly discourage all-attacker configurations because with those ships able to take more fire, their attacker partners can last longer and do more damage. That would potentially make the fights longer.

I think a 25% - 50% reduction to damage across the board is a bit too much. 10% - 15% would be a bit more palatable to me.

https://www.games2gether.com/endless-space-2/forums/65-general/threads/28848-opinion-combat-too-binary-in-a-spatial-time-sense?page=1

I feel like the stats of these two configurations nicely sum up the problem, especially with late game scaling:

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Left: hyperium shields and defense modules, +9k shields and +1k HP for a ship that already has 30k HP,  +25% defense against lasers, nice I guess. Right side: Pure offense, hyperium intensifiers. Damage doubled, attack power with critical hits quadrupled. This ship has 3 times the HP, but more than twenty times the attack power of a defensive Guardian with the same weapons (10k HP, 1.35k attack power).

I think critical hits not ignoring defense, and intensifiers not increasing critical hit chance, would both be a nice start in balancing offense and defense out a little.

I think intensifiers should be scaled down too... even in a perfect situation (pure armor vs pure projectile), defense modules at most effectively double the HP. Intensifiers on the other hand can more than double the damage output in any situation, plus add critical hits. Especially late game ones are simply too good on offensive ships.