Does the following not work for you?
kabob menu > New Window

If I right click with mouse on tabs I see this - no new window button (like Firefox, Chromium & etc..)

If I try to click in the KDE Menu, I have a start new instance button, but it doesn't work

I had this problem with Fedora + KDE too, with older versions of Brave
Just in case if you need to see, here is a screenshot from Chromium with right click

The New Window item is in two places. Please see the screenshot below:

wow I never clicked on that '+' with right click, who would suggest there are these options. That way it's working. Could you add the other buttons too like in Chromium?
Let say I have opened a tab and I want it in a new window. There is no way to click 'move the tab in new window'. The only option is to click that '+' and copy/paste the address. Also the new window button in taskbar would be very useful. Also a button when right click on a bookmark toolbar.
Let say I have opened a tab and I want it in a new window. There is no way to click 'move the tab in new window'. The only option is to click that '+' and copy/paste the address.
The "Move to tab to new window" functionality is planned to be implemented by version 1.0.0 (#4402).
Also the new window button in taskbar would be very useful.
I'm not sure how this would work with KDE, but there are issues for Windows (#6094) and Unity (#6887).
Also a button when right click on a bookmark toolbar.
Could you please clarify what you mean? I'm not aware of any options missing in the bookmarks toolbar. Screenshots of the current options:


Great catch! I filed issue #8063 for that option.
I'm closing the issue. Thanks