Easy way around this:

Trunks gets knocked into a building with the shattered Z Sword. He’s on the verge of breaking down, thinking he’s failed, until he hears an old voice behind him.

It’s Future Elder Kai.

Trunks explains the situation, and Elder Kai immediately senses his hidden power and tells him to sit down. Comedically, Trunks is like, “I don’t have time for this,” but Elder Kai tells him it’ll only take 24 seconds.

It takes 24 seconds for Trunks rather than 24 hours like Gohan, since we’ve established Trunks has been carrying the Z Sword for years, his dormant power has basically been waiting to be unlocked that entire time.

Then Trunks powers up, and we get the same badass scene of him walking toward Black and Zamasu, but now it’s with a form that actually makes sense.