Horizontal space after staff

is there a way, to make a staff not stretch the whole width of the page, like you sometimes see in the last staff?
I want to write some excercises and want it to look like this, for better overview:

There are about 20 excercises, so I would need it quite a lot

As long as each of your exercises is a separate flow, it should just be a case of setting Layout Options > Note Spacing > Justify final system only when x% full to something relatively high.

You can do it on an individual basis using the Note Spacing tool in the left panel of Engrave mode. Find and select that tool, click on the square handle above the final barline and then use Alt-Left (you have to use the keyboard) to pull it left, or Cmd/Ctrl-Alt-Left for larger increments.

A thanks that’s it!
Doesn’t work with mouse?

Nope. If I remember correctly, it’s for technical reasons - each time you nudge something in the note spacing tool, Dorico has to recalculate the spacing of the whole system. If you could do it with the mouse then Dorico wouldn’t necessarily be able to keep up with you!
In this particular instance - a single staff with a very regular rhythm - I guess it would be fine, but in the case of e.g. a thick orchestral texture…