I have a few posts upthread on my recent experience travelling from Brussels to Wuppertal via Cologne on Wednesday 4th December.
ES9132 from London arrived in Brussels slightly late, and as the convenient short cut to the platform for Cologne connections is currently closed, we reached Platform 8 just in time to see ICE317 depart.
Eurostar stamped our reservations to permit travel on any suitable later service as long as we checked with the train manager before boarding.
We joined ES9459, which was running through to Cologne but it arrived 25 minutes late and lost further time en route. We eventually left it at Rheydt to continue to Wuppertal on local services, changing at Monchengladbach. If you read upthread, you will see that we would have reached our destination earlier had we changed at Aachen
Returning yesterday, Thursday 12th December, we took the hourly local service from Cologne Hbf to Aachen, where we joined ICE16 to Brussels. It was held about 25 minutes for connecting passengers from other local services who rushed to board. (There are more frequent local services from Cologne that involve changing en route). Aachen Hbf has lifts to the platforms, so luggage is manageable, but there are no escalators, so a tight connection is likely to be stressful, although it did seem that the ICE was held long enough for everyone who used the lifts.
In the event, we had plenty of time to make our London Eurostar at Brussels, although that eventually departed late owing to an IT problem at Passport Control.
My advice is to stick.with DB rather than Eurostar from Germany to Brussels, but the DB schedules can chop and change somewhat day by day, so you do need to keep checking. On Thursday, ICE315 had been rescheduled to operate about 2 hours later than usual from Cologne (Ehrenfeld rather than Hbf), but on the day it reverted to it's original timing. The meant the online live departure board, which shows revised departure times of delayed trains in a white box, showed a revised time for ICE315 of 2 hours in the past!
If you make reservations for your full itinerary for that day (including non-reservable local services) DB will email you should rescheduling make any connections cease to be viable.
Good luck!