Hi,

all Nightjets services of Austrian Railways ÖBB was on sale since a month on their website. 
Were did you checked that the trains are full? The route Amsterdam/Brussel-Vienna is very busy. Alternative Routes are to take the IC Amsterdam-Berlin and from there Nightjet to Vienna or IC Berlin-Vienna (only seats) or exit in Hannover and take there the NJ Hamburg-Vienna. Also possible to take some ICE and IC trains of Deutsche Bahn that runs over the night. 

For the other nighttrains to Bucharest or other places is booking opened yet? It would be strange that the trains are full. If the booking is opened should be possible via ÖBB website (for nighttrains that start/ends in Vienna, also when you get on board for example in Budapest). 

I would suggest to book directly via ÖBB via their website oebb.at using one way ticket option and as discount card Interrail Global Pass. 

Amsterdam - Munich on 9 Jan. has availability in all comfort classes, as far as I can see. Check on tickets.oebb.at.
There is no Brussels - Munich night train; you can take an ICE or Thalys to Cologne though and there change to the night train coming from Amsterdam.

Budapest - Brasov on 15 Jan.: I think reservations for the new timetable (starting 11 Dec.) are not open yet and timetables not complete yet. There are currently up to 3 direct night trains and 2 direct day trains Budapest - Brasov, and the night train coming from Vienna (Budapest Keleti dep. 22:45) can be booked via tickets.oebb.at.
The same is true for Bucharest - Vienna: reservations for the new timetable don't seem to be open yet. This night train can be booked via tickets.oebb.at.

Hi, sorry I did write for Nighttrains Amsterdam-Vienna, but you are searching for trains to Munich. My bad.

There is one daytrain connection on the Route but the Nighttrain connection have a beautifull name called “Corona” (Already before Covid ) and even have a nice Hungarian dinning carriage. Bestwise you book via a experienced Ticketagency :)