I’ve got the Eurostar into Brussels and then got the train to Walibi several times, it’s super easy to do.
Walibi has its own train station which is literally across the road, accessed by foot bridge.
In terms of crowd levels, I believe that Belgium is now entering school holidays, but the queues never get too crazy there. Most are unlikely to top 50 mins, or maybe just over an hour for Tiki Waka due to its low throughput.
Fast track, if you did want to go with this option, is pretty good value. You can buy an all day unlimited fast track wristband for €45 which gives you unlimited fast track access to all the major rides, except Kondaa which you only get one ride on. Very good value, but for some reason it can only be purchased inside the park. We got ours from the gift shop at the park entrance.
We aren’t normally big fast track users, but given the affordability and unlimited nature of the wristband, we figured we’d be mad not to on this occasion.