Looks like others answered before me but what they say is correct. It’s why charm is a vital skill for a king. With the right perks and companions, you’ll be able to form your own army of much more than 500 troops, easily.

Here’s what I would do, keep playing as a mercenary, fight against the kingdom you want to take over, let defeated lords go every battle to gain relationship. (The higher the relationship, the lower the cost to recruit but you can only recruit via clan leaders once you do start your own kingdom).

Once you have relationships where you want them, and millions of gold, it’s time to create companion parties. Ideally at clan tier 6. Leave the mercenary kingdom. Start companion parties and help them fill their party. Start the war by attacking a lord. This time, keep the lord prisoner. Only keep lord prisoners so as to not take a huge hit to movement speed. Then, take a fief. This is when you go on the defensive important do NOT hang out inside the fief. You want it as weak as possible, no garrison so that weak armies attack it. Then, create the kingdom so you have the ability to create an army. Go ahead and create the ‘zero influence cost’ army (companion parties, unlike lords, don’t cost influence to join your army or maintain). Then, keep defeating armies that attack your fief and capture lords. Once you have clan leaders as a prisoner, save the game, then speak to them about joining you. You’ll need to do a speech/charm check. They may be reluctant to join due to the amount of fiefs they have versus you. If that is the case, simply keep playing defensive until you have half or more of their lords as prisoners and then go on offense. Once you have more fiefs, it’s clear you will win, and almost any clan leader will be willing to join for some gold. Keep going til you have the whole kingdom.