Is attacking a huge advantage

It is a significant advantage of the fleets/ships are roughly evenly matched.

I would imagine that it also depends on the offensive and defensive capability of each ship. There will always be some first-mover advantage unless the attacker cannot harm the defender (in which case they have bigger problems), but the magnitude of the advantage would depend on attack vs defense.

In a battle between high attack and low defense / hit point ships attacking is a huge advantage because a lot of damage can be caused per round. By comparison if each ship is largely defensive, with high defense and hit points, but poor weapons the advantage would be slight. Also the symmetry of each side would matter (as Frogboy pointed out).

It makes a bigger difference on something that will be a short battle than a long one. What's important is how much of the total damage is done in the first round.

For example, imagine two fleets of small fighters with 6HP, 4 points of offense, and no defense, 3 ships in each fleet. In this case, it's quite possible that the defending fleet is going to loose a fighter before it gets to shoot back. Two fighters are going to have lower odds of destroying a fighter, so the next time the attackers shoot, they have a decent chance of killing another ship. Basically, even though both fleets are identical, the defenders will almost certainly have 3 times the losses of the attacker.

At the other extreme, imagine two fleets of medium ships with 16 HP, 8 points of offense and 8 points of defense, 3 ships in each fleet. It's rather unlikely that a ship will be lost in the initial round. In fact, there's a 17.8% chance that no damage will be inflicted, in which case the advantage goes to the defender. It's much less likely that the attackers will survive with two or three ships intact than in the previous example.

Absolutely! Going first gives you one firing round to wreak havoc before your opponenbt can retaliate. The more damage you can inflict in that round (as a proportion of the total enemy hit points) the better attacking is. Its also worth noting that if the enemy consists of many small ships the advantage will be bigger than if fighting a few large ships (but I believe that this was your point popup target).

I've been playing around with the concept of making fleets of glass cannons. Ships with almost no defense, but huge attack numbers. As long as I attack first, and I have at least the same number of ships as my opponent, there's a good chance they'll all go boom before they can hit me.

I also make tiny ships with 0 defense as escorts. The idea is that since the ratio of attack/defense is used in determining which ship gets hit first, these small cannon fodder will be shot first, but my main capital ships (the real cannons) who have a small bit of defense will be lower priority.

The only issue with this strategy is that 1) Weapons cost more/space than armor, and 2) Need really good logistics for this to work well.