The setting you're probably looking for is live view.
E: I don't know if there is a setting to open the aperature for more light while framing/focusing, but it seems like you could do that manually by either shooting in apeture or manual mode?
Live view will show you the image in live, but 'adapted' to the settings you're using. So it will be too dark.
In the past you had camera bodies that let the aperture in full until you shoot, and had a button to preview the depth of field (a button that closed the diafragm to the aperture choosed).
If you shoot in aperture or speed priority you will enjoy a bright image in the screen even if you choose f11, so technically its always possible to have the same in manual mode....
But of course nothing of this works when you're using an external ring slave flash like the metz.
You are looking to turn the "Live View Display" setting off. When it is set to be on, it tries to approximate the exposure you will get out of your shot. When this is off, it will give you a viewable image even if you're under exposed with the current settings.
Here is a dpreview post with the same problem and same solution: https://www.dpreview.com/forums/post/55512947
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