I can add...
You are correct that there is no option for reserving seats, both these trains are essentially mass transit. I have never run into a situation that there were no seats, maybe near commute times they fill up, but not the times you are looking at. If you are really nervous about it, then maybe spring for first class, most trains I have been on, even if the car is crowded, the first class section is all but empty.
You should also know that you are essentially buying the right to travel that route that day. Even though specific trains are listed, you are free to travel on any train on that route, that day. So if you miss a train, just hop on the next one, or if you get to the station early, and have an earlier option, hop on. The only restrictions is if you get on a Eurocity Direct train, you need that ticket for that segment, or you need to add a supplement. Of course you can't use Eurostar trains either. You could even stop in say Brussels or Antwerp for a few hours, then get on another train and continue your journey.
As for best option, you actually are going out of the way by going through Brussels but only one change, the shortest route is Bruges-Ghent-Antwerp and onto Rotterdam, which requires two changes. If you look at your travel day, there is a 9:58 train that takes this route, and takes a half hour less than your train. So what is best? Saving some time or saving a train change?
