“We know J.J. Abrams is preparing one now that I will hopefully do next year for him. I look forward it. It will round out a series of nine, that will be quite enough for me.”
That sounds like he's definitely done at least with Star Wars trilogies. He's been the man behind the music for them since 1977's A New Hope and may feel that a fourth trilogy is so far off in the distance that he's unlikely to want to do it. That said, he could still contribute music to the other Star Wars movies, like he is for Solo: A Star Wars Story, and movies that have nothing to do with Star Wars at all.
Williams is an Oscar nominee again for his The Last Jedi score, making it the 51st of his storied career. Incredible.