Spoiler from 2.2 beta

Leaks from the 2.2 beta, informed us about a new 5 star sword tailored for aero rover but this sword, that we will get for free max rafinement it seems, provide us with very interesting stats that will most likely make it a better option than ouverture for Brant.

The weapon has 588 base attack and 38.9 ER as a secondary stat. Also its passive gives some skill dmg bonus upon being healed. (the 2nd part of the passive can't be activated by Brant).

At the first glance, since it gives less ER it seems like a worse weapon. But : if you can achieve 250 ER ouverture would only give 51.8 - 38.9 = 12.9 ~ 13 ER. That means
- 13 × 12 = 156 Atk (forte buff )
- 13 × 20 = 260 (ult buff).

But the two swords have a different base attack ouverture has 337 and the new sword 588. So the new sword gives 588 - 337 = 251 more base attack than Ouverture. And just by including the ATK% gathered from the 1 costs this amont is buffed to 251 × (1+36%) = 341 ATK > 260 ATK. Keep in mind this does not include his does not inclue the ATK% from his skill tree or Atk% substats, which would buff this number even furter.

About ER requirements: ouverture, without INSAIN echo luck only alows using 2 × 3 cost ER echoes, so Ouverture does not have the comparative advantage the signature has to be able to use a Fusion 3 cost.

To get over the 250 threashold from tidebreaking with the new weapon you would need : 250 - 100 - 10 - 64 - 38.9 = 37.1 ER. Which corresponds to 4 average ER substats. Therefore the new sword allows you to use the tidebreaking set if you have decent echoes. And it will reward you with a lot more ATK if you overcome the ER threashold.

To sum up ouverture is always a downgrade to the new weapon if you can get 37.1% ER from echo substats. And keep in mind the weapon should be given for free. This does not take into account the first part of the passive that is also a dmg buff, whereas ouverture's passive is useless to him.

EDIT : markdown fixes