Great game, I've had a lot of fun playing this. Games are long solo, and they'll take longer if you're new to it, expect 8-30 hour playthroughs. If you want to get into it now, I would recommend you buy the base game on its own and wait for a sale to get the DLCs. Now, there are a lot of DLCs since this is a paradox game that's been out for a while, so I'll run through what they all do:
-Galactic Paragons is the newest, it adds cool leaders, a couple of civics and origins centered around the overhauled leader system which came in the game version released at the same time as this expansion
-Apocalypse and Utopia: These are some of the earliest DLCs. If you play to the late game without these DLCs, you will notice that your empire feels kinda samey. Apocalypse adds planet killer weapons, which you may use an ascension perk to unlock for your empire. Having one built gives a special total war casus belli, which is very useful. Utopia adds a lot of fun things to do in the late game that might otherwise get stale.
-Distant stars: I wouldn't play the game without all the events from this DLC. You will see tons of them every game, you may memorize the common ones, and if you find one of the rare ones (like everyone's favorite Brainslugs), you will find that your playthrough has suddenly become much more unique.
-Federations: if you want to make diplomacy with other empires more interesting, this is for you.
-Overlord: If you want more flavor and benefits from vassals or even from being a vassal, this is for you. Pretty cool origin in here, which can make playing as a vassal actually pretty fun
-Ancient relics: adds fun and interesting archaeology sites. You will interact with stuff from this DLC every game, just like distant stars
-Leviathans: adds a few fun things to find sometimes in the galaxy. The name might give you a hint as to what they are. Really cool to find and fun to play around with until you have memorized them all
-Nemesis: Have you ever felt like your (insert total war empire here) wasn't doing enough? Do you feel like your crusade to exterminate all xenos has become... anticlimactic? Or has cleansing the galaxy of Xenos left you with post-genocide depression? Have I got the solution for you.
-Synthetic dawn: I know some of the robot stuff was from here, but I don't remember
-Megacorp: You can play as a megacorporation empire. It's pretty different.
-First contact story pack: more events. Not exciting, but hard to complain
-All of the species packs: Some of these are cool. They unlock civics, species traits, and origins that can mix up your games. Necroids and toxoids are probably the best done out of these, but my personal favorite is aquatics.