Before you decide anythingSend one invoice. Get back the validated document.
Pick one client, with that client's agreement, and send one real invoice in whatever shape it exists today: a spreadsheet row, a template, an export. You get back the same invoice as an XRechnung or Factur-X that passes the published rule sets, in the profile that client's recipient needs, with the validation report beside it. No charge, no obligation, and nothing is stored beyond the work.
How it is done today, plainly: by hand, by us, within one working day. The validator and generator that will make this an endpoint are being built and are not open yet. That changes nothing about what you get back, and we would rather tell you than let you picture a pipeline that does not exist.