The screen flexibility is there.
I make use of all three menu Locations
In addition, I replaced a few icons (used in the tag browser) with a reduced size version
Some Items , I just use Hot keys, so there are no Icons wasting (placed)
valuable screen space. You may have noticed, I not only leave the cover browser closed, but I run it detached (and usually on my second screen).
The nice thing about Calibre's visuals: If you don't like where it is , change a setting and try something else
(Most are under Preferences: Interface. a few I use (like dates, are in Preferences: Tweaks).