I've never used any of the paid DLC stuff. I might at some point, but it's certainly not required by any means.

Lots of totally free mods, though.

Edit to add: The paid DLCs add new mechanics to the base game and are basically the same as old school expansion packs were for offline games. It's not like that lootbox bullshit or anything scammy. Also, these DLCs are ongoing development. Game's been out since like 2015 and they just release a new DLC last month.

Also, I'll add that I am not affiliated with Colossal Order (developer) or Paradox Interactive (publisher) and this is not a paid advertisement. I just really like the game, just not enough to pay for DLC.