Why is the username change once every 30 days?
Probably to prevent people from changing them super frequently, which can be confusing for other users. This is pretty standard for a lot of online services that let you change your username at all.
But if you really “need” to change your username, just make sure it is avilable and then feel free to contact a moderator, we’ll change it for you.
Honestly, I wouldn’t mind if it was limited to once a year or something. Let people take advantage of it for the use case where they don’t like a name they made years ago, and otherwise stick with the username.