Well, once you view the standalone screenshot window ONCE, it should remain as a standalone, minimizable window for the rest of the session, even as you take new screenshots.
Now just turn off that post-capture pop-up dialog completely and let it autosave and stay minimized.
Now whenever you take a screenshot it will be in the standalone window which you can move, minimize, resize, etc.
No need to use the post-capture popup or the main sc window.