In the typical planetary system there are these things called worlds, moons, asteroid belts gas giants and suns. So the system space is very very vast. And given blind spots and time delays on radar and electronic sensors there is a lot of room to sneak in for an attack.
Once you turn your drive on, you're not "sneaking" anywhere. The singular advantage a defender may have is burying their ships in orbits mixed in with civilian ships, with their systems "quiet", so as to mix in the noise of civilian activity -- assuming you have a volume of civilian activity. However, turn on your drive to maneuver, or ping a "bogey" for a sensor lock, the enemy fire control is going to light you up like a christmas tree. The defender could also have dedicated, orbital sensor platforms that can go active and had locks off to other nearby vessels. Of course, the invading fleet can have these "scouts" as well.
Any battle here needs must take into consideration the fact that this is 3D environment. And thus ships will be in battle formations and will attack in formation. This is especially true of fighter/attack craft--when they are going to go after larger ships they needs must work as a squadron or their attacks will be lack effect on larger ship with their sizable armor.
Yea, not really.
First none of the game systems model combat this way.
There is simply ALMOST no value to a 3D environment in space combat games. 3D combat makes complete sense for terrestrial dogfighting, because the 3D aspect adds a component of momentum that's important to dog fighting that simply doesn't exist in 0g space.
Next, the systems that model this at all, model the combat over enough time that the ships can roll on their axes freely, and even change facing freely (to a point), which eliminates any particular advantage one angle of attack may have over another angle of attack outside of the 2D plane.
So yes there are very much room for flanking attacking. In fact--the flank attack are the very kind of attacks the fighter/attack squadrons will make... they will not charge directly in rather they will often fly at right angles of the target and then once in a position to attack at a flank or the rear they will go in for the attack.
These aren't dive bombers over Midway. They're attacking from 10's of thousands of kilometers away, over time frame of 20-30 minutes.
They will target engines and weapons and launch/recovery facilities. They won't kill a cap ship or cruiser but they can show it down and reduce its weapons. Hence this is why defensive fighter screens and escort screens are very important line of defense. But they can't be every where.
Yea, save that none of the combat systems model THAT either. Be happy you can hit the ship at all from 30-60K Kilometers away, much less a door or an exhaust vent.
Also, given that any sane strategy of defense would mine (you know nice little 3D fields of 1000s of cubic miles of 5-10 ton nuke drones that are places on the key jump ports and will way-lay any ship without the right codes) the jump spots close to the main worlds to prevent any ships from jumping in and laying assault.
"Jump Port"? Anything outside of 100D is a "Jump Port". That's LOT of space. It's actually more efficient to just have a ship with launchers that can maneuver towards the threat rather than litter near orbit with zillions of systems that all need to be properly placed in orbit, maintain those orbits, plus all of the other maintenance etc. And you still need some kind of fire direction and sensor platform to light up and guide the missiles.
Also given key worlds will have both ground based and orbital meson gun emplacement, you don't put your big ships too close until you can take them out via. orbital bombing by smaller faster ships.
Anything that can kill a deep meson site can be killed by a deep meson site, since deep meson sites tend to best be attacked by complementary meson guns. The detail, though, again, still, is that having a maneuverable asset wielding the same meson guns is much more flexible. A planet full of meson guns doesn't work well against a fleet patrolling the 100D limit and interdicting incoming and outgoing traffic while the planets meson guns sit idle and out of range.
Also given SDB, missile platforms (or boats), and/or system monitors will be propositioned in key points (usually in asteroid belts and gas giants and key refueling areas to pounce on enemy units), if one wishes to secure those areas one better commit sufficient unit resources to successfully deal with those forces. Also securing the gas giant will be one key target to be able to have prolonged operations within the system and is the first stage to being able to lay siege to the core planets.
Asteroid belts are not key points, they're really, really big. And if you want to get all 3D, the invaders can come in above the ecliptic and drive straight to the home world, bypassing them completely.
Static defenses let the invader beachhead in open space. Who needs gas giants when they can bring tankers. The defenses at the GG can't defend the core worlds, each GG needs to be defended, since ships don't really care what GG they refuel at (so, in our case, that would be Jupiter, Saturn, Neptune, and Uranus). Sure, the outer planets are far out. But the invader can secure an outer GG, and then Jump in system to hit the core worlds, bypassing other GGs. Similarly, the core world defenses can't support the GG defenses.
Frankly, Starfire is the only game that models these kinds of assaults in any real form. I haven't seen anything from Traveller that does this well. And the systems we have in Traveller are cumbersome enough that people don't try them.