CK3: I’m new to Crusader Kings and this subreddit. Any tips?

I know that PS5 has some features restricted to the expansion pass like the royal court. Are there any other features missing?

None of the expansion pass content has released for console versions of the game yet. People who have and don't have the expansion pass have all the same features right now.

When I get into a war, I never get any support from my allies, even though it says on the war screen that I have them at my disposal. Do I have to manually bring them in?

Yes, you have to ask your ally to join your war.

How do I reliably build my army? My rivals tend to have far larger armies than me and it’s stunting my progress.

Buildings, setting your marshal to organize your army will increase levy size (the better his martial skill, the more there will be), your own martial skill gives higher levies as well(your spouse can also do a council job that raises your martial skill). If you're tribal, fame gives you higher levies.

Size isn't everything though. Going down the martial lifestyle perks can make your army much stronger. Buildings can make your MaA be able to fight troops much larger. Having good knights and martial lifestyle perks for more of them with higher damage is really strong. In combat paying attention to things like the terrain you're fighting in, etc. can turn a battle around.

You also need to take advantage or your allies, and maybe save up money for mercenaries if you need them.

How do I build a stable economy? I know holdings are important but I never seem to be able to make enough gold fast enough to have a good amount of holdings to develop my economy.

Be sure to use your steward to increase development in your capital and try to have 100% control in each of your domain as well (you can use your marshal to increase control). If you're tribal, you need to raid for money. Building in your domain is a slow process though. Expanding will also naturally bring in money over time, but you can also play tall and try to build up a smaller area through buildings alone. If you're Catholic, see if you can ask the pope for money.

How does succession work? I know there’s laws you can put in place to make succession work for you but my domain always gets split up. Is there a succession law that works best?

You're basically stuck with partition laws in a lot of places in the beginning of the game until you get innovations late in the game to change to something else. You can look here to see how the different partition laws work. Some cultures get special laws that they can apply to individual titles though (you can apply title laws by clicking on the shield for a title and there will be a button to change its law). Irish and Gaelic cultures will have the option for tanistry, some other cultures on the British isles and other places will have the option for Saxon elective, and some Scandinavian cultures will have the option for Scandinavian elective. The Byzantine Empire will already start with primogeniture, and some other places will start without partition as well.

A lot of times you can also disinherit children or ask them to take the vows in some circumstances to knock them out of succession. If you have partition, try not to have two titles of the same rank or your realm will divide on succession. If you have confederate partition, titles may be created on succession, so if you hold one kingdom but 51% of the de jure area of another, that kingdom will be created on succession and that area will be lost to you on succession.

/r/CrusaderKings Thread