Hello! Unfortunately there is no such feature in Xournal++ yet. One can only move a pdf page, or delete them. As Xournal++ works on top of the PDF document and keeps the original file AND the .xopp file, one can change the PDF with an external program. Its best to apply changes to the PDF first and then start to annotate on the file in Xournal++.
As long as one does not change the order nor does not delete pages in the orignal PDF chages can be done at any point, as the page numbers won't be affected. So rotating pages should be fine at any time.

Being able to rotate pages or the window as a whole would be a great feature.

This still doesn't work and it's a rather basic feature of a PDF editor. There seems to be a 'Paper format' option under 'Journal' but it's greyed out. How come this still doesn't work?