For posterity, I found the problem. I had “PORT = 3000” set in my .zshrc profile, which must have overridden the Strapi environment settings. I commented it out with a “#” , and then loaded a new terminal instance and checked that “PORT” wasn’t set using the “env” command.
I then ran “yarn strapi dev” in the strapi folder and it worked as expected, with the admin panel opening at “localhost:1337/admin”.