I'm betting she was created in late 2019. She's inspired by Oscar from Rose of Versailles, something that Oda was really surprised by and only just finished reading in September 2019.
Also you've got her hair. Something that's ripped straight from the iconic two-toned hair in kimetsu no yaiba
KnY only started popping off in late 2019 which matches up with the Rose of Versailles comment. In other words, Yamao was potentially created around the Udon section in Wano.