Yes, Holly mentioned at an event that there will be more. She also said that if you read the ending of The Prisoner's Throne, it’s pretty clear what’s coming next, and it’s from the POV we’re all expecting—not Nicasia’s. She confirmed it will be a direct sequel.
SPOILERS for The Prisoner's Throne: At the end of The Prisoner's Throne, Cardan asks Oak if he can delay his abdication for “just several months” to put a backup plan in place for protecting the throne. He also mentions Cardan and Jude’s upcoming journey to The Undersea.
In the Waterstones edition, Holly talks about working on the Undersea adventure in her Q&A.
So, based on all of this, it looks like there will be a book or series following Cardan and Jude’s adventure to the Undersea, and it’ll be from Cardan’s POV.
As for the 2025 release date, Holly hasn’t officially announced anything yet, so it’s likely just a projected date.
This is the video of the event: https://www.tiktok.com/@dominika_kuruczova/video/7424956071845956897