Hi - given your changed plans well worth a read of seat61 relevant pages for travel through to Germany, if you have not prev done so
https://www.seat61.com/Germany.htm
Also note what he says about booking to 'London International CIV' from your UK origin point.
I'd book the eurostar that has good connection to DB ICE from Brussels (eg the 11.04 from Sty P which I took last week to Koln) - works well.
Book with DB website if you can (the ticket got e-mailed to me - can show on your phone and print off too - which I did as a back up). DB held the connection at Brussels as we were slightly delayed in.
By booking on DB website they have a good seat selector tool for your reserved seat on the ICE, which was useful.
If you don't like the DB booking site I'd try Loco2 (but they charge a modest booking fee) - it's a very simple site to use so you may consider that worth it (but DB is pretty simple too)
Seat 61 also has pre set links to DB site for the trains he recommends (so the journey is all set up for you as it were, just click 'buy'), but if you need onward destination beyond that you could alter it, or if you have time at change of trains point in germany go tot he ticket office or a tickt machine for your 'last leg'.