There are some .env settings that are pretty much only useful if you are managing multiple users or Joplin Cloud itself. The only reason I ever turned some of them on was for beta testing before Joplin Cloud went live.
There is a thread which has some discusssion about the mail server and makes reference to the config.ts file which shows a list of .env variables that can be used. Note that this is an older version of the file and makes reference to MAILER_SECURE which has been replaced by MAILER_SECURITY.
Some of the .env variables are:
MAILER_ENABLED= Enables the mailing system - 1 is ON 0 is OFF
MAILER_HOST= SMTP server address
MAILER_PORT= SMTP server port
MAILER_SECURITY= SMTP server security method - none or tls or starttls (Server 2.7.4 and above)
MAILER_AUTH_USER= Login username for the SMTP server
MAILER_AUTH_PASSWORD= Login password for the SMTP server
MAILER_NOREPLY_NAME= Display name for mail sent by the server
MAILER_NOREPLY_EMAIL= Sender email address for mail sent by the server
SIGNUP_ENABLED= Enables site visitors to sign up from the server login page - 1 is ON 0 is OFF (I have not used this since before Joplin Cloud went "Live")
TERMS_ENABLED= Enables the login page terms and conditions link - 1 is ON 0 is OFF (I have not used this as anything other than "OFF" since before Joplin Cloud went "Live")
ACCOUNT_TYPES_ENABLED= Enables the admin to set user accounts as Default, Basic or Pro with their associated usage limits / sharing features - 1 is ON 0 is OFF
There are some others such as USER_CONTENT_BASE_URL= but the last time I looked at a more recent version of the config.ts file this was commented as not being in use. Others appear to be specifically Joplin Cloud related.