One good contribution would be, if you looked into the current abilities of ERPNext in detail and created a report of where it falls short of meeting the requirements of German Finanzamt and Tax Advisors. We had a session with our tax advisor this week and everything looked okay. But we only looked at how sales invoices will be booked. That was alright.
Just saying that your impression is, that it is weak, is not the best way to contribute to a change. An analysis with specific requirements would be “gold”.