Amazing to read all this and good that your attention has been drawn to a flying legend, at least in Holland .
In Nederland we have very few pre-war rebuilds. One of them the Lambach-HLII will probably never fly again, due to certification problems.
You obtained a homebuilder’s permission, but I suppose that in Danmark you will also need control and supervision by a governmental office?
All the luck with your project!

Adri

Off-topic: I rebuild a wooden Ka-6 glider, but the supervision and certification is not comparable with a motorplane (much easier).

Very off-topic: I was involved in the design and construction of the "Svanen", the heavy lift vessel, that took part in the modular construction of the Storebælt-and Øresundbridge. The Svanen is a giant spaceframe, but the strengh calculations wil in principle be the same as with the fuselage of your D-XXI.
My time in Nyborg and Malmö and the contacts and discussions with the Scandinavian colleagues were very enjoyable.