I really like everything you’ve posted! I’m a big fan of your clean painting style, you’ve inspired me to do my Rebel’s similarly to yours. I especially love the theme of fleet trooper helmets and colour scheme throughout, it’s my favourite colours for the Rebels, and I always adored the fleet trooper design and image of them lining the sides of the corridors to fight the storm troopers blasting in.
Anyway! Awesome job! 

You should never take him without his Armament upgrade. His pistol configuration is fantastic (and usually preferable to the sniper rifle) since he usually has the aim token necessary to activate Long Shot and make it shoot to range 3. There are very few lists I wouldn't start and end with it on the Pistol side while going up against.

I'm on verge with Crack Shot - my experience with Cassian is nowhere near that with Sabine, and I personally feel much more comfortable with Explosions. This is a personal choice, however, and either is good. SoS is a given, a huge Luke's activation can be game changing. 

I would agree with changing Last Stand to My Ally is The Force, as it is much less situational and gives you nice spread of two cards per character, which helps should one of them die prematurely. I really like Symbol, and it fits nicely with the rest of the list that rather lacks support against suppression.

The 3pips are a good choice, Luke's cards are either outdated (RotJ) or better suited to lists where he's the sole focus (I am a Jedi). If you had a spare, 4th trooper unit a Covering Fire might be a consideration as it can generate more tokens than Legacy.

Well, she has her quirks  Most importantly:

Sabine does not aim. Sabine does not dodge. Sabine does not recover (unless for free). Sabine either moves in and shoots, or shoots and move out. 

She has Spd3 and Jump - utilize it! All the roofs, balconies, tree branches, statue heads etc. are yours, so stay on'em - not many things can catch up to you there and they might sometimes negate enemy's cover.  While we're at it, try not to target stuff in heavy cover. Look for easy pickings (light cover at best, possibly no immunity to pierce or imperiousness) - this way you can get to 1-2 wounds inflicted per shot , and that's what you want. Sabine is not a finisher, she's an aggressive piece to engage with. She has other to pick up what she left behind (like Cassian). Her damage is usually spread out among many units (bombs!), but when you sum it up, it's huge

This may be risky, as she will get shot at. She wants to recover her shield with her 2pip nad Cassians 3pip, she want's dodges from Luke's 2pip. She forces the rest of your list, especially Luke, to be more aggressive than usual, as you want alternative threat to her (well, there is also Clan Wren, so they might be useful in this capacity).

Oh, and don't get caught in melee (remember the "roofs" part).

If you're lucky   Luke is 6 dice, Impact 2 and Pierce 2, Cassian is 3 dice, Marksman and Pierce 1, Sabine is 3 dice, Pierce 1, K2 is 5 dice. All surge to crit. If you're super lucky, you get 17 hits with Pierce 4 against 11 hits on ATST, which statistically blocks 6 - yeah, that's ATST down. 

Good luck getting all crits though  

Much better strategy to deal with the ATST with a list like this is just slaughter everyting else. Sabine decimates red-diced stromtroopers, and her Explosions! card helps with those pesky troopers as well. Luke hunts the commander, Cassian and K2 keep in check units not currently occupied by the two heavy hitters. Then just hide (if Sabine, Mandoes and/or Luke are alive, that's should be easy thanks to Jump) and get whatever the objective is on the last turn.

Cassian with the pistol, K2, Luke and Sabine can probably take out an AT-ST in one turn.  You'd have to line it up right and be really careful on the approach.
I use Cassian's pistol as a really long stick. I can reach out from Range 3 and kill people, even if they're hiding in the bushes in cover.  He's a crit machine. You get enough dodges and he can be almost invincible as well. K2 is  a trick piece.  He's actually super tough and resilient but you can't shoot at him until he's already too close. You actually want people trying to kill K2. Jyns pistol can be a nasty weapon with a couple of aim tokens, but if they are shooting at K2, Luke, Sabine and Cassian can get really close and murder units

Yes it can work.  Try this, its super fun blasting fools!
 
GUNSLINGERS!
++  Standard (Rebel Alliance) [783 Points] ++
+ Commander +
•Cassian Andor [110 Points]: A280, Offensive Push, Overwatch, Recon Intel
•Han Solo [139 Points]: Endurance, Environmental Gear, Improvised Orders
+ Operative +
•Chewbacca [103 Points]: Offensive Push, Tenacity
•Sabine Wren [167 Points]: Electro Grappling Line, Personal Combat Shield, Situational Awareness, •The Darksaber
+ Corps +
Rebel Troopers [40 Points]
Rebel Troopers [40 Points]
Rebel Troopers [40 Points]
+ Special Forces +
Rebel Commandos  [48 Points]: DH-447 Sniper
Rebel Commandos  [48 Points]: DH-447 Sniper
Rebel Commandos  [48 Points]: DH-447 Sniper
+ Command Hand +
Command Hand: Sorry About the Mess, •Crack Shot, ••Last Stand, ••Symbol of Rebellion, •••Change of Plans, •••Notorious Scoundrels, ••••Standing Orders
+ Battle Cards +
Condition Cards: Clear Conditions, Hostile Environment, Rapid Reinforcements, Supply Drop
Deployment Cards: Battle Lines, Danger Close, Hemmed In, Major Offensive
Objective Cards: Intercept the Transmissions, Key Positions, Recover the Supplies, Sabotage the Moisture Vaporators
++ Total: [783 Points] ++
Created with BattleScribe
You can do other stuff splitting points and bringing K2SO to the fight but bringing Chewie lets you double control the order phase.  I think he is better than K2.
K2 allows you to support an infiltrating cassian better however.  Also, you can bring Sabines 3pip which is good.  Switch things up...both versions solid.

you can actually fit 7 named characters into a GAR list at the moment, Rex, Kenobi, Padme, R2D2, Fives Echo and Aayla Secura, you cannot quite fit C-3PO into that list as even with only 3 Phase 1s and Fives and Echo on them it woud come out at 812 points so you have to drop 3PO
in this list only 1 units does not have a hero/named character in it, i dont think that it would be a particularly good list though
797/800 Clone Captain Rex (90 + 0 = 90)

Obi-Wan Kenobi (175 + 0 = 175)

R2-D2 (35 + 0 = 35)

Padme Amidala (90 + 0 = 90)

Phase I Clone Troopers (52 + 36 = 88)
--Fives (36)

Phase I Clone Troopers (52 + 40 = 92)
--Echo (40)

Phase I Clone Troopers (52 + 0 = 52)

TX-130 Saber-Class Fighter Tank (170 + 5 = 175)
--Aayla Secura (5)

I've run Han/Luke/Chewie/Sabine set a few times, it's risky but really fun to play. Also, finishing your games in under an hour is really cool

Current version is sth along the lines of:
 
Hero Army
794/800 (9 activations)
Commanders:
 - Han Solo (120): Duck and Cover (4) = 124
 - Luke Skywalker (160): Force Push (10), Force Reflexes (10), Emergency Stims (12) = 192
Operative:
 - Sabine Wren (125): Situational Awareness (2), Personal Combat Shield (10) = 137
 - Chewbacca (95): Hunter (6) = 101
Corps:
 - Rebel Troopers (40): Medical Droid (18) = 58
 - Rebel Troopers (40): Z-6 Trooper (22) = 62
 - 3× Rebel Troopers (40) = 120
• Explosions!, • Son of Skywalker, •• Symbol of Rebellion, •• My Ally is the Force, ••• Change of Plans, ••• Return of the Jedi, •••• Standing Orders


It seems like currently the two factions that can pull off a Hero Hammer list are Rebellion and Republic, due to both having cheap Jedi option (Luke/Anakin, though they're not strictly necessary) and cheap combat oriented characters (Rex, Cassian, K2, Chewie). 

K2 without Jyns gun will struggle to get in the fight sometimes. I usually just take 3 naked corps troops, then you can squeeze in the snipers for more units. I'd also consider having a more similar theme of attacks. Cassian with the rifle doesn't hit as hard and usually attacks from long distance. Luke and Sabine and mandos are the opposite.  Either go all out on the close quarters theme and overwhelm a flank, or you may find yourself getting beaten piecemeal. 

Yup, you definitely over-equipped the Veterans  They're not all that good in hero squads, mainly because the list is starved for orders as it is, so you won't utilize Defend 1, nor will you benefit from Coordinate, as you do not have any emplacements. Cut the extra fat, and place naked Rebel Troopers in their place, you just got yourself an extra 54 points!

These can be spend in numerous ways: Jyn's blaster for K, Situational Awarness or Off.Push on Resistance, Sniper Weapons Team or extra troopers. 

I'm also not a fan of Electro Grapplig Line on Sabine, Cassian's Volunteer Mission may help with readying it, but it's use is still an action, and Sabine does not have those to spare.

Having said all that - yeah, pretty fun list, go give it a try!

PS. Have you considered which command cards to use? It can be tricky in a list like this

you have luke and clan wren with rockets so you have some anti armour capabilities but i wouldn't rely on the rockets too much, it might have impact 3 but it is only 3 red dice, most of the vehicles in game have more than double that and you can only use it once. Luke is a good option as he has surge: Crit and Impact 2 with Pierce 2 on 6 dice and he has his one pip to attack twice, you can do some serious damage against vehicles with luke (with pretty much any lightsaber user tbf)

Mind that, barring specific builds, the rebels don't usually do well against armored enemies. That said, you have enough Impact (Luke and rockets) and surges to crits to be able to handle AT-RT's. When you see sep's or republic tanks, you'd better play around them

As for the list, it looks better than previous one in my opinion. I would still make some changes, but I won't say what they are - you do you and then tweak the list to your liking