You're more than ready to go independent, so the sooner the better.
It's a coin toss whether a lord brings any fiefs with him. Frankly it's more fun when they don't, so no big deal either way.
Persuasion skill is a waste of time. It only works on the fickle lords, who are the ones you don't want. Its far more efficient to just defeat the lords you want in battle, capture them and then talk to them to recruit them. No persuasion needed that route. On the topic of lords, you don't want powerful ones, you want loyal ones. Using sarleon as the example, Brennus has knights of the lion, but is hardcoded to argumentative. You'll lose relations constantly with him once he's a vassal. You're better off with regular lords (only honorable or marshal personalities) who will follow your instructions reliably.
Put about 400 decent troops (armored longbows and men at arms for sarleon) in any fiefs you want to keep for yourself, then add 600 or so cheap low tier troops (sarleon militia). 1k total garrison is enough to deter most attacks, give you a pool of troops to train from in an emergency and keep the numbers fairly balanced if you get sieged by 2-5k of enemies later, all while being easily affordable. Meanwhile, i'd suggest taking a hard to siege fief (mobray castle works great for sarleon) to use solely as bait. Keep a very small garrison of say 50 men at arms and 50 longbows in it and you'll find most enemy attacks pick it as the first target. Join the defense when it gets sieged and you can hold it comfortably at 10:1 odds.
If you like a challenge I'd suggest avoiding the use of knighthood orders. If you get 2-300 members of a powerful order (shadow legion or knights of the ebony giblet for example) you can faceroll everything on the map so easily that all the challenge will be gone before you know it.