I would love to have my costum faction spawn randomly, but two costum faction (non-pantheon) always spawn. How to I get really random faction spawn?

I have two underground adaption faction (non- pantheon) with dragon rulers which have spawned like four to five games in row. I have over 30 non-panthoen custom factions. I like to play on maps with nine factions. Somehow the faction-selection-RNG seems to like repetition very much. Is there any way to avoid the RNG picking the same factions over and over?