The issue isn't that they're easy to destroy, it's that once they're in outer space and moving, their shields will unfold and they'll be all but impossible to destroy. Hence why the Resistance need to go and gather some more ships and allies to help them with their attack.
The plan was outlined during the 'briefing' scene at 1:36:20. I've bolded the line about their explicit vulnerability to a shot to the guns.
Poe: Once those Star Destoyers are on Exegol, we can hit 'em.
Rebel: Hit them how?
Rebel woman: They can't activate their shields until they leave atmosphere
Poe: Which isn't easy on Exegol. Ships that size, they need help taking off.
Poe: Once the tower is down, the fleet'll be stuck in atmo for just minutes with no shields and no way out.
Rose: We think hitting the cannons might ignite the main reactors.
Lando: That's our chance!