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!

I would say absolutely not. Modlists and the DLC are just the frosting on the cake. I’ve been playing since 1.17 and was straight vanilla until very recently when I snagged up Biotech because I wanted generational arcs to the stories I was playing. Not to knock mods at all. I go absolutely buckwild with mods on other games. Base Rimworld simply scratches that itch all by itself.

Note: I have played a heavily modded game with both Royalty and Ideology, not Biotech though.

None of the DLCs are strictly necessary, but I would highly recommend Ideology - the core mechanics it adds are compelling and impactful, and it serves as a nice base for a huge amount of mods to build off it. Royalty isn’t bad either, but it seems more disconnected from the rest of the game than Ideology is.

The dlc is good but not necessarily a prerequisite. Though a lot of content mods depend on assets from the dlc.

As others have said, the DLCs aren’t strictly necessary, but it’s good to have them. If you like playing modded, vanilla still has plenty of content. If you do decide to get a DLC I’d recommend Ideology, because it’s the most meta-changing DLC.

I recommend playing the base game and getting back into the swing of things, then get the DLC that interests you most once you’re hooked again. The DLC add a lot of content and can dramatically expand your play styles and experiences. There are tons of mods that can do the same for free too, and most don’t require any of the dlc. My personal favourite DLC is biotech!