Maya Phillips is GREAT, but her interests seem more vast than just theatre. I wonder if she would feel too constrained simply focusing on stage? Her recent pieces have included reviews of movies (Vivo, Shang Chi, Small Engine Repair, Free Guy), TV (Loki), art installations (Van Gogh), theatre (Williamstown, Lackawanna), and feature stories on all those artforms. She has also written poetry, and has a book about "nerd culture" coming out next year. Her current "Critic at Large" role might serve her best.
I don't know who I would recommend to the NYT as the next Isherwood. They also have a decent number of go-to second stringers (Elisabeth Vincentelli, Laura Collins Hughes, Alexis Soloski) from whom they could choose, though I hope they won't elevate one of them.