Based on what I know, here’re some ideas on how I’d set up the scenario. Beware, they may be rather complicated as I am specifically trying to make them dynamic and interesting. They can always be toned down if needed, and I’m sure we (or your players) will find ways to make it pull on an even more diverse set of skills, if such a thing is desirable. As always, I strongly suggest allowing your players to put forward what skills they would like to use in the situation.
First choice: Do you want to go straight home and try to lose your pursuers through protracted combat/speed, or do you want to try and lose them in the mountains?
(To answer your question, I would just toss the mountains in. They aren’t exactly subtle to have escaped the characters’ notice)
Over the Swamps
If they want to lose their pursuers through protracted combat/speed, then they basically have two options: Go just out of range of the AA defenses and fight it out, or make a beeline and open up on the Strikers with the turret.
This is a moot point if they don’t have a turret, but:
The catch on trying to chase a target is that if you aren’t going in a straight line, you’re losing time even if you’re going “the same speed.” Because of Star Wars shielding technology, let’s ignore drag and losing speed through maneuvers. If you zig-zag, you are covering more ground. If you go 25% faster but cover 30% more ground, you’re losing time. If you’re firing on your pursuers with a turret, you force them to either take evasive maneuvers (covering more ground) or settle in for what could be an easy shot in order to keep pace.
So, if they want to actually conduct a chase rather than a structured time combat, I would make it a Gunnery vs. Cool (Triumph or 3 Advantage kills an enemy fighter) or Gunnery vs. Piloting (Planetary) (Triumph causes the pursuers to suffer a hit for base damage). Add a catch that with Piloting (Planetary), the TIEs count as moving one speed lower than they really are (to a minimum of 1). It’d be up to you as a GM to choose what the NPCs do, and it could vary from round to round. For example, if the PCs succeed in shooting down a Striker while the TIEs are using Cool, maybe they switch to Piloting (Planetary).
Once they’re out of range, it becomes just Piloting/Piloting, with exceptions I’ll mention later.
Talk of Skills
Now TIE pilots, unfortunately, lack both Cool and Discipline. There are two ways to rectify this. One is to give them one of the two skills (Discipline, to contrast with the Rebellion’s Cool would make sense), the other is to make the check Piloting as well, and simply have the effect trade-off. More easily shot down, or more easily outrun? After all, since it’d be a minion group the skill has no difference anyway.
Also, you’d have to decide if the TIE Striker is (Planetary) or (Space). I’d choose (Planetary), in which case you’d need to give TIE pilots (Planetary) as a group skill, but ultimately it doesn’t matter since they’re NPCs. Just know that their dice pool is YYY for Agility skills, and YYG/GG for everything else, whatever skills you assign them
Mountains:
Now, consider the other alternative: Flying to the mountains. This will require actual piloting skills, and navigation with the Survival skill.
Piloting/Piloting through difficult terrain (number of Setback up to you/player choice/roll results) is the obvious, but then once they’re a certain range away (say, past Close), now make it Survival/Perception. But what’s the difficulty there? Up to you. There should remain upgrades, but when you aren’t having to also dodge laser blasts you can take the route more easily. Survival to navigate and either already know the mountains or guess the layout, Perception to try and follow the ship through the “sensor blind” region by either seeing them blip in and out on the sensor screen, or by actually viewing where they went.
For Survival, I’d make the difficulty set based on how treacherous/dense/complicated the mountains are, upgraded by half silhouette in the same manner as Difficult Terrain. Setback can be added for visual conditions, unfamiliarity, etc. Despair can result in being forced to fly above the mountains, immediately becoming visible again.
For Perception, there would be two difficulties: One based on everyone being down in the mountain range, and one with the pursuers above the mountains (when above the mountains, the pursuers always use Perception).
If the pursuers are down in the range with the pursuees, then they have one less difficulty than the pursuees, but an additional Setback from any visual conditions. If they’re up above, then they count as moving one speed faster (for resolving chase checks) and ignore Setback, but increase the difficulty once.
As always, Despair can result in a collision when appropriate, and can also be spent to force either party up out of the canyon.
Patrols:
Now, the patrols. If the PCs are Short range or more ahead of the TIE Strikers, have the patrols use Gunnery against the PCs’ Computers. A Triumph or 3 Advantage inflicts a Critical Hit on the PC’s ship as a missile detonates nearby (consider it a proximity fuse, rather than a direct impact). Computers allows the PCs to detect ground targets and/or spoof missiles, allowing the pilot to perform fewer evasive maneuvers, while the Gunnery from the ground fire slows down and damages the PCs, allowing the TIE Strikers to catch up.
From previous rolls (e.g. Computers in the Imp base), perhaps the PCs catch wind of these patrols. In that case, Knowledge (Warfare) vs. Leadership of the Imperial commander could be used to avoid the patrols, similar to navigating terrain, or the PCs could decide to go through the unpatrolled mountains instead (or, instead of unpatrolled, make it “patrolled by patrols light on AA”).
Conclusion:
Hopefully, the PCs would succeed in shaking or destroying their pursuers and escape back home. Successive Triumph and Despair could indicate such an eventuality, beyond any damage or range, and even perhaps they think they escape only to discover that the Imperials shadowed them and learned of the location of their base (this would require the Imps to have stronger sensors than the Rebels, or to be able to mask their own presence while staying within sensor range themselves).
If they take the mountains, increase the amount of time it takes to get back by 100-200%. If they avoid the patrols, increase the time it takes to get back by 50%. If the time it takes to get back doesn’t matter, then just bear this in mind for the narrative.
Edit: I forgot to add, but you could also have the TIE Strikers open fire, making it an inverted version of the turret section I mentioned earlier. The catch is that if the PCs want to choose to fight it out with the turret gun rather than running and trying to dodge fire, they simply enter combat.
