Most efficient is to fly to Vienna as it's the farthest east. Then work back west. Munich works, but you need to travel even farther east by train to Vienna and then double back.
But of course figure out which flights are best for you - cost, schedule, etc, and then work from there.
I'd travel east to west because of slightly higher chance of good weather in the low countries at the end of the trip.
I'd break the Salzburg to Brussels train day with a stop either in Strasbourg France or along the middle Rhine castle zone. About halfway, and worth two nights. Both are on the high speed train lines.
Also note that there is a NightJet train from Vienna to Brussels. You could get a sleeper car. Salzburg then is a issue, but the train from Salzburg to Linz Austria (a NightJet stop) is just over an hour, direct, and frequent. You could easily leave Salzburg at check out time, arrive Linz, bags at station, look around Linz (which is a nice city!), go to dinner, then 10pm onto train. Sleep the night in a private sleeper cars, and be in Brussels at 11am the next.
Only caveat some people don't sleep well on the train.
Also check out staying in either Ghent or Antwerp in Belgium - both quite near Brussels and both probably preferable tourist cities.