Whats seems to be a trivial task - renaming the pi user - is not that easy anymore, when you have detailed look on it and search the internet for ways to do it.

Apparently a simple usermod is not enough, because there are a lot of references, where pi is not renamed.

Unix etc. has very detailed description of steps to manually rename the user.

Even better there is a script renamepi that automatically renames the pi user with the trick of putting all the things in rc.local (temporarily) before pi processes are started.