Since XRechnung will become mandatory in Germany for many governmental customers of companies, the use of Konik would be great to simplify software development across all existing e-invoicing standards in Germany. Does it already - or when will it support XRechnung/ZugFERD 2.1x?
As far as I'm aware, Konik does not support ZugFERD 2.1 yet.
However, Mustang does. Note that I'm the author of mustangproject.
I keep a list of open source projects on https://zugferd.github.io/.