Signed up for Equifax to freeze my credit, password can not be longer than 20 characters
Not only does the credit bureau max out their password length, you have a small list of available non-alphanumeric characters you can use, and no spaces. Also you cannot used a plused email address, and it had an issue with my self hosted email alias, forcing me to use my gmail address.
Both Experian and transunion had no password length limitations, nor did they require my username be my email address.
Username and display name can be set independently, you should have a "Display name" field in settings. Their non-unique display name is "max" and their unique username is "@kittykittycatboys@lemmy.blahaj.zone". If you check their profile you should see both.
If you don't set a display name it will be the same as your username, if you set display name to the same as username (like I have) it'll show your username without the instance even to people on other instances.
If you open the profile it seems to have two names. I have the feeling that only one of them is valid with the instance postfix, despite it being shown with both