Travel from Spain up to Brussels?

It's not an unreasonable distance over the course of 4 weeks. Train travel time from Barcelona to Brussels along the fastest route is 9 hours with a change of stations in Paris, or you could build in a bit more time for that Paris connection. But if you were going direct, not stopping to see Paris, then you'd usually fly for less money.

Next, the question is whether Paris and other stops along the main line are part of your sightseeing plan, e.g., Montpellier, Avignon/Arles, Lyon, Dijon. Or, you might fly to Nice and then train toward Marseille, more of Provence, and the points above. Other train routes abound, of course, but take a bit longer, e.g. if you wanted to go via Carcassonne, Dordogne, and the Loire valley.

Looking Up Train Schedules and Routes Online gives you the DB train schedule link and tips for using it. For flights, try www.skyscanner.com. If you don't have Rick's France guidebook, then get some initial ideas with links at https://www.ricksteves.com/europe/france.

Two possible routes in my mind:

(A)
Barcelona > Costa Brava > Provence > Lyon > Burgundy > Paris > Brussels

(B)
Barcelona > San Sebastián > Bordeaux > Dordogne > Loire > Paris > Brussels

It's plenty of time and you've gotten several good options. If you figure about a week in Barcelona (and region) and a week for Paris, that leaves 1-2 weeks for the trip between Barcelona and Paris and then Brussels/Belgium.