If you join a planet-spanning civilization with the dictator, there is a malus but your people will become part of the other culture anyway.
By the way, when people are killed after you failed to eject the dictator, it appears that it‘s never the dictator who gets killed by these events (dust particles, rough landings, earthquakes etc). Bug? Or is it done on purpose?
It could be a viable strategy to try to get rid of the dictator…