This seems to be a core problem as the same issues can be observed with the default standard print format in our test-instance on frappe cloud, which is completely stock.
Thanks for the help, unfortunately this has no effect. I abandoned the print format builder and I’m using custom format now.
The margins and font-size are now working, but I am getting nowhere with the font-family issue. I installed Helvetica on the server and it is showing up in fc-list, but wkhtmltopdf is still defaulting to DejaVuSans.