The first day I would build
Give the city one clear route before adding extras.
- Start with Speicherstadt and Elbphilharmonie while energy is high.
- Use Miniatur Wunderland as the natural reset instead of crossing town too early.
the easier plan is Speicherstadt and HafenCity first, Elbphilharmonie or Miniatur Wunderland in the same block, St Pauli later. That keeps the day readable instead of turning every good name into a separate detour.
I would rather leave one place for tomorrow than drag a tired route through St Pauli just because it looked close on a map.