You groom them from children. This is an unpopular opinion, but it’s the best solution I could think of. Shamans have done the same in tribes. Some children show empathetic and leadership skills, stading behind the weak and sharing things equaly among siblings and other children. You pick those and groom them from children to take on the burden to be leaders. Yes, this is not fair, they’ll never grow up to choose what they want to be, but so are so many things in life. Sacrifices have to be made for the greater good… and so many far worse things have happened in human history.