What should I do? Maybe just be a vassal and help one of the factions takeover calradia?
There are a few methods, first the easiest use Mods and Cheat until the kinks are sorted out.
Key Mods
Party AI Overhaul Commands
DiplomacyFixes
DrasticBattle
SmithForever
BannerLord Tweaks (increase the size of your army based on Steward and Leadership)
True Prisoners
Workshops Work Much Better
Tournament Fair Armor
True Noble Opinion
True Relations
Fun Mods
Separatism
AI Marriage
Custom Spawns
campaign.give_troops aserai_mameluke_guard 30 (sturgian_berzerker or battanian_falxman are also excellent)
campaign.give_troops battanian_hero 30
campaign.give_troops khuzait_torguud 30
campaign.give_troops vlandian_knight 30
With the army above + good strats you should be able to beat an army twice your size or greater. Basically set your Infantry up in a line. Set your archers behind them on a hill. Skein formation your Cavalry and hold them while sending your horse archers in ( I use F3 charge). Let their troops come to you and send in your Cav. Bring your Cav and Horse Archers back after the 1st wave rinse and repeat.
Now, with Party AI Overhaul Commands you can control your parties and add them to an army (without a Kingdom I believe ). This lets you double or triple the size of your army. Setup your parties with Companions with high Tactics.
For your first settlement, focus on something out of the way like Aserai, maybe Tamnuth Castle. Get the settlement when you are unaffiliated and it is easier to keep. Or use the commands below.
campaign.give_settlement_to_player Vostrum
campaign.add_gold_to_hero 1000000
You could also go the Merchant route, get your Trade to 225 and get the Everything has a price perk. You can buy a settlement this way. Takes a long time though and I found it tedious. Or you could try to take a settlement without cheating.
Game still takes a looooooong time to finish even with cheating. Purists may not enjoy my approach, but I've loved it. The game has been amazingly fun. I like to cheat though, 100% automation of games like Everquest etc.
I have 738 hours played thus far.
Enjoy!