1. I’m not entirely sure if a pass will be the better option here. Compare prices of the global pass with ordinary tickets. Check ordinary tickets via the website of Deutsche Bahn, they have a good deal with Eurostar. So, tickets booked via Deutsche Bahn are cheaper, than those booked elsewhere (e.g. Eurostar, or B-Europe).
2. You might want to combine ordinary tickets with a One Country pass, Germany. Note that these One Country Passes (Germany) passes are also valid from Brussels! (But only when you take the ICE to/from Cologne/Frankfurt from there!)
   Or you can save a bit on Eurostar, if you book London-Aachen or Cologne as an ordinary ticket via the DB website.
3. Don’t take the Thalys between Brussels and Cologne. Their reservation fee is extortionate and costs almost as much as cheap advance tickets.
4. If you decide to go for a pass, be aware of the mandatory seat reservations on Eurostar. Check availability on the Eurostar first. There are only limited places for pass holders on the Eurostar, so they can sell out. Since it’s Easter, demand probably will be higher than usual. Reservation cost is about 30 EUR seat reservation per person, single journey.
   Booking seats can be done here: https://www.b-europe.com/EN/Booking/Pass#TravelWish, for which you need a pass cover number:

    

Many Thanks Brendan