DLC are only really mandatory if you are playing with mods that require them.
The base game of Rimworld is still chock full of stuff to do and stories to experience, and i always reccomend people play just the base game by itself before thinking about DLC or mods, because DLC adds so many mechanics and new items as to be completely overwhelming even if you have a familiarity with the base game, and if you have no familiarity? Good lord!