I wouldn't sack her out just yet, well I don't sack horses anyway, but she's so young I wouldn't want to be putting too much pressure on her joints and muscles yet. If she spooks with the sack on her back, she could hurt herself if the dead weight twists her spine or something.
I would concentrate on making her ground work perfect
in every situation, and get her moving from pressure. If she knows ground work like turn on the forehand, back, turn on the haunches etc, take her out for in hand walks and ask her to do stuff like turn on the forehand in a strange environment.
Teach her leaving any buddies and the safety of the yard isn't scary, and she can trust her human handlers. Get friends to come visit her and get her used to comotion and people laughing and not being toatlly horse-sane around her.
You could take her long reining, but idk if you have bitted her yet? What have you done? How far have you gotten?
Foals learn a heck of a lot in their first couple of years in this world. Things that happen now will be ingrained in her brain so keep everything light hearted, fun, safe and friendly