Biggest problem in the base game was there were lengthy periods in which you fast forward the game cause there was not a lot to do.
I'd get Way of Life and Reaper's Due first because they flesh out the game and let you play it as an rpg-lite.
Reaper's Due adds a bit of difficulty because characters seem to die a bit more than in the base game, but I think its a decent trade-off for the detailed injury system
in which you can cut off hands from captured foes and pagans can sacrifice their eye to please the ancestors, leading to the Baltic Sea area being inhabited by a bunch of one-eyed cunts.
Sons of Abraham adds a bunch of stuff to dick around religion-wise, so I'd pick it up along the way perhaps together with Legacy of Rome which is also somewhat minor but seriously QoL stuff.
It's a pretty dick move on behalf of Paradox, half of this shit should be added in regular patches.
So, it would be Way of Life, Reaper's Due, Sons of Abraham and Legacy of Rome - at this point the game feels feature complete and playable.
Monks and Mystics adds some more stuff to do, mostly centered around secret societies and artifacts. Artifacts were a pretty nice addition, but the societies are a bit hit and miss as higher ranks
have some overpowered stuff that may be too borderline supernatural. Not to mention that one time when the King of Pomerania joined the Assassins and went shouting allahu akbar from the highest rooftop in Szczecin.
So yeah, it's a bit derpy at times, but overall I like to keep it on even though I don't always use it.
Some folks say Conclave is the shit, but for me it felt like it adds too much micro to the game that isn't really needed at this point.
Charlemagne, in my view, was mostly attractive because of the earlier starting date, but it doesn't change too much unless you want a very long and epic games.
Rest of the stuff pretty much boils down to whatever cultures you would like to try and which parts of the world you'd like to see more fleshed out.
I'd say Old Gods was the best of the bunch so it's great that you already got it. Read up about the other stuff on the wiki:
https://ck2.paradoxwikis.com/Downloadable_content