Becoming Leader of the Southern Empire

Hey everyone this is my first ever run on Bannerlord I downloaded the game roughly 3 weeks ago and I've been loving it! I own 3 fiefs, just crushed the rest of the Western Empire last night and have gotten much much stronger than any of the other lords in the Southern Empire including the current leader Ulbos (Rhagea died in battle like 2 in game weeks after I started, slaughtered by the Aserai 😭) I'm at the point now where I want to take over ownership of the Southern Empire entirely before I begin what will essentially be a crusade against the Khuzaits. My relations with all the other clans in the Southern Empire is at 100, so how do I go about taking over? Any advice or tips are appreciated!