Currently, around ⅔ of the available space is wasted on tablets.
On my tablet, the grid view looks like this:
Landscape mode
Portrait mode
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There is https://github.com/nextcloud/android/issues/912 and https://github.com/nextcloud/android/issues/1742 so there have been discussions in the past but it is not a priority of any developer at the moment.
and well… your own issue 
I’m talking about the grid view of pictures only.
Can’t you make a grid view for a pure picture folder?
In my opinion, that would be quickly and easy to realize, isn’t it?
It actually would be rather complicated and tricky. Since we would have to handle this completely different within showing file lists that have a 2-pane layout on tablets. It really is hard to achive imho 
Is there an alternative app for just displaying photos out of Nextcloud?
LMGTFY, there is https://play.google.com/store/apps/details?id=it.linfaservice.cloudgallery but I can’t say if that’ll fit your requirements
That app does not work.
It worked when I tested it briefly.