My idea is give new users a heads up about the "username situation"
I've joined Reddit recently and on my entire sign-in and onboarding flow I never got a hint that usernames cannot be changed in the future. Or that my username will be generated by an AI or something. At least a hint so as to let a person know that if they decide on the username, they can change it and stuff like that.
I'm sure just adding one or two pages, or let's say a subsection on a page, during the onboarding to tell the user about all of that, then hiding it in the terms of service or privacy policy that no one reads, could be better