Re: own domain and use a unique LHS
"however I'd still like to be allowed to choose a username instead of being forced to use the email address as my username"
The extreme worst case is a site-issued username generated from other data such as a concatenation of real name and DoB (yes, I have a site that uses that).
Keepass will generate passwords that look like line noise. Perhaps a useful addon would be an option to generate usernames, preferably pronounceable ones.