The script for the Flash sequel has already been completed.

Despite The Flash's protracted and problematic development, a sequel's script has already been finished and is set to be released in 2023.
A Warner Bros. Discovery official claimed in an interview with The Hollywood Reporter that David Leslie Johnson-McGoldrick, the writer of Aquaman, had already completed a story for a sequel to The Flash, assuming it "goes well." The Flash is scheduled to come out on June 23, 2023, as at the time this article was being written. It appears that the DCEU is attempting to bring its greatest stars back to theatres with Aquaman and the Lost Kingdom also being postponed until 2023, a follow-up to Matt Reeves' The Batman in development, and Dwanye Johnson pushing for Superman to face Black Adam.
Considering how dangerous the production of The Flash was, the news of a sequel still surprises people. Prior to choosing Andy Muschietti as the film's director, The Flash underwent a rotating door of writers and directors, starting with Phil Lord and Chris Miller and ending with Rick Famuyiwa. If the movie manages to release in 2023, it would have taken an incredible eight years to make. Hollywood talent representatives claim that the infamous Warner Bros. Discovery merger "poisoned the DCEU well," which makes the situation worse.
Will Ezra Miller Appear in the Flash 2?
Ezra Miller, the star of the movie, is frequently in the news for allegations of burglary, harassment, abuse, grooming, and kidnapping, as if that weren't enough. Miller faces a maximum sentence of 26 years in prison if found guilty of the current burglary allegation brought against them by the Vermont Superior Court. DC is still confidence in The Flash despite all of this, as seen by their commissioning of a sequel story.
Miller has now expressed regret for their unpredictable and perilous behaviour, and he was spotted returning to the Warner Bros. set for The Flash reshoots in early October 2022. A WBD report implies that the movie's final cut has also been decided upon. This is encouraging for the likelihood that a sequel will be approved because filming could only begin once The Flash is published. In light of this, the fact that the script for a The Flash sequel has already been written may hasten its creation. The sequel to The Flash may avoid the hilariously slow production process that plagued the first film.