Here's an example of how generalizing space-combat is difficult. The nature will depend on technological capability, extremely so. For example, if you can shoot missiles at, say, 10km/s that leads to a different type of combat than if you can do so at 100km/s, or 1000km/s, etc.
Highly relativistic missiles, at say 0.98c or more, are effectively impossible to defend against, and massively potent. At that speed a kinetic "warhead" massing only 10 milligrams (perhaps a millimeter in diameter or less) would have an explosive yield of nearly a kiloton. More so, when the missile is 15 million kilometers away (40 times the distance from the Earth to the Moon) the time between when the target detects the missile and impact is only one second.
Even at technology levels far below that level missiles can still remain extremely effective. At long distances (thousands of kilometers away) the missiles would engage in evasive maneuvers to avoid being shot down by speed-of-light weapons. And then at some point they would release a huge number of evenly spatially arranged fragments. At a missile speed of about 40km/s (which is a reasonable estimate for speeds achievable by cutting edge and next generation propulsion technologies today) you get a ratio of warhead mass to TNT equivalent explosive yield of about 190:1 (meaning that a 1 kg impactor yields 190 kg of TNT equivalent in kinetic energy). Which means that a single 500 kg missile could break up into a thousand fragments, each of which is only 3 cm in diameter (if made out of depleted Uranium) and has the kinetic energy punch of nearly 100 kg of explosives. If, for example, we imagine a crewed ship capable of accelerating at, say, 5 gees (~50 m/s^2) and we imagine a fairly small ship that is only about 100 m^2 in cross-section then an attacker could fire a barrage of only 8 such missiles and fragment at a distance of 400 km and have an effective 100% chance of at least one fragment hitting the target. Even with high powered lasers it is no small feat to destroy a 3 meter target at 400 km distance. As the speeds of the missiles go up they become more and more effective and difficult to shoot down.
But accelerating a kinetic warhead to .98c is just as much scifi as saying 'shoot them with phasors'. It's not something we can do with current or foreseeable tech (as far as I am aware).
The other thing is that the faster the missiles go, the harder they are to aim. You lead your enemy's ship and fire the missile, they make a minor course correction in the meantime, and your missile has to detect it and change its vector to match.
Space combat is one of those fun things to discuss where no matter what you suggest, someone will come along and advise you of something you've forgotten.
.98c weaponry is certainly scifi for us today, but it's possible that some time in the far future the requisite technology will exist. Although in practical terms it's probably easier to create .98c bullets than actual, steerable missiles (which is even more challenging).
However, your point about the difficulty of steering a .98c projectile is a little out of place. The point of shooting someone with a weapon that can travel that fast is that you can catch them with their pants down. Such a weapon effectively travels at 50 times the speed of light from the perspective of the target. This is because it is racing any light or signal which would give the target warning of its presence. For example, by the time a ship has had warning that such a projectile is an entire astronomical unit (the distance from the Earth to the Sun, 150 million kilometers) away it will only be about 10 seconds until it hits the ship (the light will take about 8 minutes to travel, but in that time the projectile will cover 98% of that distance, and by the time that light reaches the target the projectile will actually only be a little more than 10 light-seconds away). And such a projectile need only be a few microns in size in order to unleash the explosive power of hundreds of kilos of TNT, so you probably aren't going to detect it at all. So all you have to do is wait until the enemy is sitting in port and you blow their ship up from across the solar system.
Of course, weapons such as that fundamentally change the whole nature of warfare, so speculating about them is problematic.
"it's possible that some time in the far future the requisite technology will exist."
And it's possible that in the far future we'll be able to warp space, extract massive amounts of zero point energy, and more. In other words, I think it's a cop-out to hand wave about technology for this discussion by saying that it can be done in the future.
"The point of shooting someone with a weapon that can travel that fast is that you can catch them with their pants down."
The problem with this proposal is your target can easily solve this by randomizing the thrust enough, on the assumption that it might be targeted. Suppose you are 1 AU off and fire at where you expect your target to be in 8 minutes. The difference between 0.01g and 0.0101g is 100 meters after 8 minutes, so even a 1% difference in thrust might be enough to miss the ship. But if you're using firing things at 0.99c then your enemy can likely manage better than 0.01g.
You mentioned "all you have to do is wait until the enemy is sitting in port". That "port" is a space station, in orbit. Neptune is 4 hours out. If the station is a 1km sphere, then it only need to move by up to about 20km in any direction to make the odds of being hit be less than 1:100. Neptune is 4 hours out. 20km/4 hours is 0.0002m/s/s or 0.00002g.
For reference, the ISS orbit decays, due to air resistance, by about 90 meters per day. This is easily restored through occasional boosts. Which means that you, as the enemy, are going to need to fire off thousands of these 0.99c bullets in order to hit your target. Where does all of this energy come from?
In any case, with micron sized bullets, you're just going to drill a hole through your target. The exit hole will be pretty much the same as the entrance. Very little of the energy will be deposited into the ship, and it's not likely to take damage anywhere near to the amount of energy you put into trying to hit it.
Highly relativistic missiles, at say 0.98c or more, are effectively impossible to defend against, and massively potent. At that speed a kinetic "warhead" massing only 10 milligrams (perhaps a millimeter in diameter or less) would have an explosive yield of nearly a kiloton. More so, when the missile is 15 million kilometers away (40 times the distance from the Earth to the Moon) the time between when the target detects the missile and impact is only one second.
Even at technology levels far below that level missiles can still remain extremely effective. At long distances (thousands of kilometers away) the missiles would engage in evasive maneuvers to avoid being shot down by speed-of-light weapons. And then at some point they would release a huge number of evenly spatially arranged fragments. At a missile speed of about 40km/s (which is a reasonable estimate for speeds achievable by cutting edge and next generation propulsion technologies today) you get a ratio of warhead mass to TNT equivalent explosive yield of about 190:1 (meaning that a 1 kg impactor yields 190 kg of TNT equivalent in kinetic energy). Which means that a single 500 kg missile could break up into a thousand fragments, each of which is only 3 cm in diameter (if made out of depleted Uranium) and has the kinetic energy punch of nearly 100 kg of explosives. If, for example, we imagine a crewed ship capable of accelerating at, say, 5 gees (~50 m/s^2) and we imagine a fairly small ship that is only about 100 m^2 in cross-section then an attacker could fire a barrage of only 8 such missiles and fragment at a distance of 400 km and have an effective 100% chance of at least one fragment hitting the target. Even with high powered lasers it is no small feat to destroy a 3 meter target at 400 km distance. As the speeds of the missiles go up they become more and more effective and difficult to shoot down.