bbPress.org

use the bbpress logon widget in a sidebar on your forum page, and it’ll stay on the forums.

edit : sorry just read your entry again, and that’s exactly what you are doing !

I disable dashboard for all my users – untick “show toolbar when visiting site”

If they logout using the widget (which is their only option if they don’t have toolbar), then they don’t get the WordPress login page.

You can customose the generic login form if you want to keep it see

https://codex.wordpress.org/Customizing_the_Login_Form