Here is an update, regarding the FDCs.
Redently I stumbled upon an auction with the info "only 2500 FDCs in total issued":
See here 25 rote
167 gelbe
333 blaue
667 grüne und
1308 schwarze
I contacted the seller and they said, the info came from the official collectors-info of the Austrian post (they have a dedicated phone number for related questions). I couldn't quite believe it, since that would mean there are less black FDCs than red normal stamps. So I tried calling them. After several attempts I was able to get through today.
The info I got was as follow:
Usually approx 1/4 is sold as first day issue letters (FDCs). However due to the enormous demand of the regular stamp this time a lot less FDCs were produced. Unfortunatelly they couldn't give me exact numbers.
But I also couldn't get the 2500 confirmed. So my guess is something in-between ~1/8th to 1/20th maybe.
If anyone can come up with more solid numbers, that would be great. However it seems proper accounting is near to non-existant, so not sure if we will ever get solid numbers from them.