Are you flying from the US for six days?
If you use the search box, you will find that lot of posters here find Brussels not as rewarding as the smaller cities in the area. While it is not necessary to switch hotels (you can take unreserved, cheap trains for daytrips from a hotel you like ... ), you might consider whether Antwerp, Leuven, Lier might be of interest. I personally would not spend three nights in Bruges, but multiple posters here consider it the most beautiful and romantic place in the world. I do not.
Check March weather:
Bruges March 49° / 41° 10 days rain
Antwerp is much bigger than Ghent or Bruges, and has a major art museum that recently did a major renovation, KMSKA. There are several other fine museums, and lots of interesting architecture to see. Think about special plans for Sunday and Monday, due to museum closures and a small number of Sunday-only sights, like (for example) the interior of the Antwerp or Leuven city halls, or the abandoned village by the new, major Antwerp commercial port.
I agree that the Resistance Museum (and adjacent deportation theater) are must-sees in Amsterdam. The unified Ancient/Modern art museum in Brussels can not really be seen in one full day, and has a massive amount of important art. Two examples: The Death of Marat, and The Fall of Icarus. Keukenhof is a must-see if your dates include it. It does not really take a full day, and there are special bus services from Amsterdam and Leiden, if not even more places. When you visited Amsterdam, did you make any daytrips, like to Delft, Den Haag, Leiden, Utrecht, Alkmaar, Amersfoort, or others. Again, cheap unreserved local trains.
https://community.ricksteves.com/travel-forum/belgium/day-trips-from-brussels-768c7c85-75b9-4374-ad13-ae36967bc521
https://community.ricksteves.com/travel-forum/belgium/day-trips-from-brussels-besides-bruges-and-ghent
https://community.ricksteves.com/travel-forum/netherlands/day-trips-from-delft