(I'm going to put one feature/issue in each comment to make it easier to discuss each topic individually, hope that's ok)
Known Issue: Probably a lemmy-wide issue. When new users sign up, there's no error message for "username has been taken", "username too short/long" or any other exception. You have to do trial-and-error to figure out how to get past the sign-in screen. Some sort of exception handling and user message would help greatly.
Immediate workaround would be to put something on the signup page to explain this ("username has to be between X to Y characters; if you see a spinning icon forever, chances are the username was taken already")
(I'm going to put one feature/issue in each comment to make it easier to discuss each topic individually, hope that's ok)
Enhancement requests: Not sure if this is better addressed lemmy-wide or here, but: New users may not know the difference between instances. Would having the "Communities" tab default to All instances be better than Local instances?