Just in case OP is still looking for help two weeks late, or someone else wants tips, I've done over 20 Christmases in Germany now and have it down to a fine art . It sounds a lot, but I do it bit by bit over quite a few weeks, and I like baking.
I do my cake in October according to the Delia Smith recipe. I just count up the total amount of dried fruit and make that up with Corinthen, Sultaninen, Orangeat, Belegkirschen, getrockneten Pfirsischen. I feed the cake once a week with either Glühwein or Kinderpunsch. Ready rolled marzipan is now commonplace, and this year Dr Oetker is producing ready to roll fondant icing for the first time. Or you could make your own royal icing easily enough.
A friend makes my Xmas pud, she gets suet from the English shop in Cologne (mail order).
Cranberries are available fresh from most supermarkets, so we do the sauce a few days before Xmas; it only takes 10 minutes to make.
I made a large stock of custard last week, using the Delia recipe. It gets frozen until Christmas. However, I do order a can of Ambrosia too for emergencies.
I order a pack of Paxo sage and onion as a back-up, but my main stuffing is made with normal minced pork and fresh sage, then also frozen in portions.
I order a fresh organic turkey from our local Biosupermarkt - their orders have to be in by around 5/6 December. We only pick it up on the 23rd or 24th and store it in the cellar or shed to keep it cool without taking fridge space.
Maltesers and After Eight are available in most supermarkets. Dr Oetker produces small cartons of double cream.
Everything else I order from the English Shop, but I think there are expat shops in most big cities (Berlin has two branches in Charlottenburg and Kreuzberg). Each year I get a box of crackers, a Terrys Chocolate Orange, a box of Roses, spare mincemeat, Mr Kiplings mince pies, some British craft beer.
"Is the shop Britishfoods Englishbooks still in Hamburg? The owner had a second branch in Berlin for a while, which closed,"
The one run by Robert? The Berlin branch was utterly bonkers, down in Neukölln. My kids used to love going there, but it was so often empty.