IF and just an if, not a suggestion, you could add Hamburg Germany and you could make it a reasonable train trip. about half way for the 11 hour train trip.
But yeah, Copenhagen is not easy to get to from anywhere other than Germany by train.

Why Brussels? We visited Brussels, Ghent, Antwerp, and Bruges. We liked Ghent more than the others but a couple days in any of those cities are fine. ANd if you can somehow stop and check out the Antwerp train station, it is spectacular. (And the restaurant Elfde Gebod (The 11th commandment) is quirky, good food and music) https://elfdegebod.com/

In Amsterdam, I highly recommend spending time in Vondel Park (their version of Central Park) and De Vondeltuin outdoor restaurant therein. https://devondeltuin.nl/?lang=en

Visit a brown bar, preferably in Leidseplein square.
If you like beer, one of the best beer bars, all Dutch beers is Proeflokaal Arendsnest. Well known. https://www.arendsnest.nl/en/enjoy-b...dutch-brewers/