I agree. While I've not seen much uptake of e-Rechnungen in my own client circles, it's coming. It's great that Cakedesk can output an XML-Rechnung, but despite all the information out there, I think most (of my) clients would still prefer to receive an actual PDF over a dataset. And Cakedesk can produce such nice, self-designed invoices, too :-) Being able to output a ZUGFeRD-compliant PDF/A would seem the perfect match.

PDFs should be generated in PDF/A format to be ZUGFeRD-compliant (PDF/A

Yes! I would be happy to have this format.