Yamato having a DF is kinda contradicting atm.
She clashed against Ace? Not using it.
Attacked Ulti? Not using it.
Clashed against Luffy? Not using it.
Fought armored troops? Not using it.
It's not like she is a Bleach character, who oftenly holds back not to use their bankai for reasons.
She is OP characters, who always quick to use their DF power.
If Oda saved it for "big DF reveal" later, it would be contradicting.
Watch after the reveal how she will use her DF everytime, even against fodders.
But prior reveal, she is not gonna using it for mystery sake.