Well several things would need to happen:
more representation in the Assembly
Willem doesn't need to antagonize the Catholics by placing education completely under the State by
the institutions of state need to be more spread out than in OTL, removing the dominance of the Dutch
more Belgians should be allowed into the officer corps, giving the many southern soldiers their own officers instead of the Dutch/French/English officers they had
tariffs need to be placed on the cheap British products which were undercutting the Belgian products
Don't make Dutch the official language in the south, do it slowly by making both French and Flemish the official languages in the region
and open up the Scheldt, don't levy a toll for any ship passing through, allowing the strategic port of Antwerp to slowly regain its position of power as a major harbor.
The United Kingdom of the Netherlands would be the greatest of middle powers in Europe, a great economical powerhouse just below Great Britain in power due to the ports of Amsterdam, Antwerp and Rotterdam.
In the present time it would probably be just behind Germany in economic power and a member of the G7