The big moment is when the Good Guys find the Bad Guys' planetary base. Then the excitement runs hot as they determine the Star Destroyer fleet will be unable to raise shields due to being parked in the atmosphere and are therefore as vulnerable as they would ever be. To the fighters!
Did I miss some explanation as to why the Bad Guys chose this Sitting Duck Strategy? A few kilometers more altitude would put them in vacuum and englobing the planet in a full-strength defensive orbit.
It couldn't be for the purpose of hiding the fleet, since the entire planet was lost and unknown to the point the whole movie involved finding the secret map/route to the place.
(as a secondary question/observation: When the "Peoples' Fleet" arrived, the fighter-sized craft could rely on speed over shields to survive. However, the capital-sized ships should likewise be unable to raise their shields in atmosphere. The carnage would have been brutal.)