I prefer a TL that is faithful to the original.
Even if what's added fits in the story/plot/situation, it's still something that was not said, and sometimes the way a character talks and express themselves is exactly how the author wants us to perceive that character.
In this case, the sentence "tomorrow you will see everything with new eyes" gives a much different vibe than "tomorrow is a new day", even if they CAN mean the same thing.
The first gives the impression the character has a way of expressing themselves that might be "deep" and "metaphorical", while the second doesn't, it just exposes the fact, tomorrow is a new day, leaving the implications of what that means to the listener(Phos) and the reader(us).
If Lapis is meant to not express herself that deeply and be more direct, using the first changes her characterization.
It's not just a matter of how something sounds better, if it fits, or of mere preference, it's a matter of changing the way a character express themselves.
I'm all for using better translations of words if they sound better in the context in english while leaving the same meaning, but if something changes the vibe and feeling of what it's being said, it doesn't matter how better it sounds or how much better it fits, a more exact translation is preferable, because, maybe, the sentence not fitting perfectly in ta given situation might be the effect the author wanted.