You could take an airport transfer. If you book it in advance it will be about 80-100 euros. At that time of the year on a saturday, it will take about 45 minutes to get there. It's more expensive but it will be more convenient. If there is a problem with the train you'll be too late.
Belgian railways aren't the most reliable in the world. Mechanical problems, trains not riding, trains too late,... these things happen quite frequently. If you need to be somewhere, it can be quite stressful.
I wouldn't take the 6h40 train as you'll arrive less than 2 hours before your departure. Brussels airport can be sketchy though. It would not be the first time that there is a sudden strike.
To be fair if it was up to me. I'd take a late train the day before and I'd stay at an hotel nearby the airport. If you depart at 9h30, it will be a lot easier.