So I'm not the most experienced person in this game... I readily admit that I use TweakMB so I don't have to put up with all the reputation swings with the lords (I swear, they are moodier than 13 year olds!), but I've found that the best route is being a merc or just skipping that whole thing and just doing quests for individual vassals to raise rep, then starting your own kingdom.
I like to start up on the far side of the map between the Kerg, Vaeg, and Sars, because I like to recruit my army from the other factions. Once you get enough training throughout your team, getting a fully decked out army is cake and you don't even need to actively fight fight anymore (still need to be in the battle because the computer is super harsh if you let the AI decide on its own) - unless you're doing sieges.
I run with 30% Swad Knights, 20% Huscarls, 40% Rhod Sharpshooters, and 10% Companions. The Huscarls stay planted in front of the Sharpshooters for Def while the Sharpshooters and Knights literally murder everything in the field. For Sieges, I run 45% Huscarl offense or Rhod Seargents Defense, 45% Sharpshooters, and 10% Companions. Seriously, I don't even need to fight or call my armies and that team has been so OP for me. I also only play on the med difficulty setting, but I'm pretty sure it's not that different since I don't even hardly take casualties.
I also think that by not committing to another country, it's funner because you don't have to put up with all the extra BS that comes along with being a vassal for that faction.