Atma only drop with a Zenith weapon. You're free to upgrade BRD to Atma weapon, go buy the BRD Relic from a Calamity Salvager, then upgrade the Relic to Zenith and use that to farm more Atma. Or if you want you can get several sets of Atmas before upgrading BRD to Atma, your choice (made a lot easier since you can stack individual Atmas to 99 instead of them not being stacked)