Hi Plucky and welcome to the forum.
This should hopefully be quite easy to fix. Buy a bottle of blue food dye from the supermarket and paint the bald spot and surrounding feathers blue. Chooks are attracted to red (blood) and raw skin. They are generally not attracted to blue. This should protect her from the other chooks developing a plucking habit. Most chooks are beginning to moult at this time of the year so feathers are loose or just regrowing. It is the worst time for a plucking habit to start.
I would remove the aggressor/s for about a week if possible. The pecking order dynamics will change and when she/they returns (only one at a time - not as a gang) the focus should not be on just one bird.
Don't take the victim out unless she is badly injured as she will return as the newbie/victim.
You have plenty of nesting boxes. It doesn't matter how many you have, they will still all prefer the same one.