If it is our first time in these cities I think you have the split right in terms of number of night in each place.

Having been to all of these cities I would cut Paris to 3 nights and have 4 in Copenhagen which is a much more pleasant place than Paris. But as a first timer wanting to do the main tourist sights you will find more of those to do in Paris.

Honestly Brussels is a bit of a dump and I would struggle to justify more than half a day there. Bruges would be a far better option and only about an extra hour by train.

Train is the best way between Paris, Belgium and Amsterdam but definitely fly AMS-CPH.