Ok, after quite a lot of tinkering – Corbett Enders’ solution I linked to above resulted in CF7 internal Server Errors (500) for me – I noticed that there was a call to the default font missing in the first of the three if statements in classes/send-pdf.php at line 724 (Current Version 0.8.4).
So instead of uploading custom fonts and registering them with mpdf (which somehow didn’t work), I just added
'default_font' => $fontPdf
to the list of configuration variables for the new MPDF.
Now the chosen Font Family in the dropdown seems to work, which will do for now.
I didn’t check with the “Font Size”-setting but I’m guessing that will be the same.