I’m having almost exactly the same issue. I tried importing the theme several times and only on the third time got a success message. The first it appears to have timed out, leaving me with a blank page, the second time I got a 504 error. When if finally “succeeded”, I was left with messy pages.
I followed your advice and deleted all the pages and images. I’m not sure how to delete the actual menu elements, but I deleted the active menus.
I just re-starting the import process, but I’m again left with a blank page. I’m concerned I may have deleted some of the wrong things and don’t want to keep deleting things and re-trying. Some help would be greatly appreciated.