One solution is that after X amount of time AFK, the game detects that you're in "hunting legendaries" mode and gives a slight boost to the probability of legendaries spawning, I love pixelmon and I've played it many times and I always quit for the same reason: in the end-game I wait hours and hours and hours in a biome for a legendary to appear and none ever do, and I have no doubt that this has happened to many players. I hope you take into account my suggestion of "hunting legendaries mode" or make a very difficult item to craft that increases the probability of legendaries spawning for a while.
thanks for everything