you need to put a delay of 4 hours, the next train after 10:25 is 14:25
Or a 2 hour stopover would allow you to leave London later (8.58 is a lot more civilised than 6.50 IMO) and still be in Berlin by mid evening. Tends to be pricier though!
Another option to consider is an overnight stopover, to break a long journey up, if your schedule allows it (I realise many US visitors are constrained for time).
Doing that in Brussels (or Liège, using ordinary train services from Brussels to Liege - perfectly comfortable, and only busy in the rush hours) avoids all the worries. Or even take the overnight ferry via Harwich-Hoek, though that involves extra changes and is slower.
And finally - you can choose your seats on Eurostar.
At the moment, for a tight connection, I'd book in the highest numbered coach you can, to be near the main exit at Brussels, because the short-cut exit halfway down the train MAY be closed for security reasons.
Www.Seat61.com is your best guide to options.