A long time ago, in the late eighties, early nineties, an idea about a potential series titled "Excelsior" with George Takei in the lead as Captain Sulu was pitched to Paramount. The studio didn't have any interest in the project and passed on it. Naturally, that led to some disappointment as Takei really wanted to do the series as did Grace Lee Whitney, who was going to be another one of the stars.
Takei still talked about this series in 2020 when he was interview by Startrek.com . He claimed that the series would have been a "huge, monstrous, galactic success," and that Paramount had "missed that opportunity." Slashfilm writer Witney Seibold noted that there was no reason why the studio shouldn't greenlight a series with Admiral Sulu as Commander in Chief of Starfleet Command as that was the rank and position Sulu had reached in David A. Goodman's fictional history book "Federation: The First 150 Years."
Seibold indicated that even though Takei is 86, Patrick Stewart was 81 when he first began shooting Star Trek: Picard season one. So age shouldn't come into play, and I agree. However, an Admiral Sulu series at this late stage of the game and in the current age of streaming would have little to no chance of success.
Patrick Stewart returned as Admiral Jean-Luc Picard, and viewers tuned in because they knew and loved Picard. They'd seen him in action as a captain. And they also wanted a chance to see the crew of The Next Generation. Essentially, he had a built-in fan base and has remained quite popular long after Star Trek: Nemesis had aired.
Captain Sulu only appeared twice onscreen, once in Star Trek VI and once on an episode of Star Trek: Voyager in a flashback scene. And neither of those scenes were enough to gain studio interest, and there's been no mention of an Admiral Sulu series until now. Guest stars, like Todd Stashwick, that have appeared on newer Star Trek series have had more fan interest than a series that was conceived over three decades ago. So odds are good this series or any other with Admiral Sulu in the lead isn't going to happen.