I want to know when to access the DLC. Anytime? Wait until the end? I want to know!
The Teal Mask is level-scaled to your team, going as low as Level 10. It is recommended to just go in whenever you feel like it! since Kitakami will be able to keep up. (Well, up to about Levels 70+, at least…) (Sources/additional reading:) Dotesports PCInvasion Ginx
It’s worth mentioning that The Indigo Disk is not level-scaled, with wild Pokémon around Level 60 at lowest, and trainers getting up to the high 80s, however, this will not be a problem at all, since you need to beat both The Teal Mask AND the main storyline first.
Hope this helped!