I'm going with Nakia not leaving Wakanda until 2019. The earliest T'Challa's death can happen is October 2023, and the earliest the bulk of Wakanda Forever can happen is October 2024. I'd like to think they were able to have some time with T'Challa before his death. The weather in the movie is Summer like and with Riri in school, I'm thinking it's either toward the end of the 2024-2025 school year or the beginning of the 2025-2026 year.