Following you can find the links to all Anno 1800 DLC on Steam. Please note that they're only compatible with the Steam version of the game.
Deluxe Pack
Season 1 Pass
DLC Sunken Treasure
DLC Botanica
DLC The Passage
Cosmetic DLC Holiday Pack
Season 2 Pass
DLC Seat of Power
DLC Bright Harvest
DLC Land of Lions
Cosmetic DLC Amusements Pack
Cosmetic DLC City Lights Pack
Season 3 Pass
DLC Docklands
DLC Tourist Season
Cosmetic DLC Vehicle Liveries Pack