[2] After Hiragana Keyakizaka46 was renamed to Hinatazaka46 in February 2019,[6] Kosaka was selected to hold the center (lead performer) position in the title song of their debut single, "Kyun".

Kosaka was cast as the main character in the 2019 mystery film The Japanese Doll [ja] (恐怖人形, Kyōfu Ningyō).

[14] As a dinosaur enthusiast, she was also appointed in the same month the ambassador for Sony's DinoScience exhibition, which would be held from July to September at the Pacifico Yokohama.

Kosaka announced her resumption of activities on March 4, 2022,[17] and returned to the center position for "Boku Nanka", despite her "mixed feelings" about the appointment.

[21] In November 2024, Kosaka ended her exclusive modelling contract with Seventeen after 6.5 years,[22] expressing her wish to be seen more as an adult woman and an idol.