The nightjets are extremely popular, and the proper bed places often sell out months in advance. The capacity is also limited, especially to Brussels (it’s only 2 sleeper cars, 1 couchette, 1 seating car). There’s a bit of a shortage of sleeping carriages.
If you really want to take the nighttrain the option via Paris seems the more viable.
If the 1st class seats of the Thalys are fully booked, you might want to reserve an ordinary TGV from Paris-Nord to Lille, and then take another TGV or Eurostar from Lille to Brussels. Or you can go via the hourly local trains from Lille to Courtrai/Kortrijk and then to Brussels (3 direct trains per hour to Brussels in Kortrijk on weekdays, 1 direct train every hour in the weekend). That option, without Thalys, will have cheaper reservations.
Interrail reservation on Thalys is a bit expensive, and Thalys often sells out too on sundays, since everybody returns from a weekend in France for the work week.
Please note that the night trains can be often delayed and you need plenty of time in Paris to switch stations. Please provide enough connection time in Paris.
If you’re not up for the Paris option, I’lld suggest the day trains. Normally you can access Brussels from Vienna in 10 hours, usually with 1 connection in Frankfurt.
