Along your itinerary, Uke Boutique in Amsterdam is probably your best bet. Neither Berlin nor Burgebrach are close to Hamburg, at least not in a European understanding. To be frank, most ukulele stores in Europe sell the same instruments as in the U.S., but at a higher price. If you'd want something genuinely European, you'd have to look at luthiers like Maurice Dupont in France, Pete Howlett in the U.K., or Tom Ziegenspeck in Germany. Or maybe a Brüko. Then there is APC in Portugal producing a large number of solid wood instruments, but of questionable quality. In the lower end sector, you have brands like Flight and Kokio which are administered from Europe, but made in China like Kala or Cordoba and others, most likely even at the same factories.