it will be dificult, but if your clan have strength more than 50% of the other clan in kingdom,
u can start rebellion alone

If you want to do it yourself, use the vanilla mechanic of leaving your faction and keeping your fiefs.

In the end, with patience and a bit of being a son of a bXXXh, I managed to start a solo secession rebellion.
It took me three attempts, and sometimes it went really badly, but here's what I did.
With the maximum number of clan groups and a lot of influence, I create an army. I enlist everyone except my groups. And I put the miserable soldiers in my group and go with recruitable prisoner forces.
I head to an enemy city and... I crash the huge army against the defending machines.
Another option is to approach an enemy siege of a friendly fortress. And gradually leave the groups from my army. They go to fight confident that the rest will help them... and... that's what I don't do.
It's really not something I like to do, it doesn't tie in with the "role" of my character, a pretty honest guy. But I can't think of anything else.

After the military disaster, I disband the army. All the groups are decimated. I recruit prisoners, so as not to be attacked by hostile groups that are nearby. The rest... they pass.

With this I have managed to start a succession rebellion alone. But, as I said, it breaks my character role. Although I understand that the game cannot do it, I would have preferred that the player could start a rebellion without this 50% of the military weight.