Wouldn't the other Powers be quite upset with France annexing Belgium? It was only two decades since the Napoleonic Wars and I can imagine them being very wary of France expanding again.
Yes and if France owned Antwerp in Belgium it would be far more difficult to supply and gather an Army on the continent to stop France, especially given how quick the round trip from Southeast Brittian to Antwerp is. What is the next really good port at that time in Europe?