Note: I'm not using Galaxy.

I have Stellaris 3.5.3, installed using the offline installer packages, and want to update to the latest version. Do I need to separately download all of the DLC installers for the latest version of the game, or does the offline patch executable automatically detect and update DLCs in addition to the main game?

bump.

You only need to install the patch.

(Not that it's ever a bad idea to backup the latest versions of the installers though)

the dlc needs to match the full game code so yes if its been a few years the odds are go that your extras will need to be redownloaded when you reinstall the off line game

if however its only been the last pact or you then you may get away with it