There’s a lot here to process, but I believe something key is missing from the calculus. The boss gets to save against Hold Person at the beginning of their own turn; for any PC going after the boss, the odds of the boss being held are more or less halved.
Moreover, if the boss goes right after the caster, Hold Person does nothing at all if they succeed either save. (Odds of failing twice are 0.6^2 = 0.36 ; odds of not failing twice are 1-0.6^2 = 0.64). My point here is I believe that the computed odds for when the boss is held are overestimated by quite some margin.
That could be the case for the poison as well, but I don’t know if or when poison triggers additional saves.
If the initial save versus poison is made then I believe there is a follow up immunity to that type of poison. So you can't just keep hitting an enemy with the same poison type hoping they will eventually fail the save.
I'm not certain how long the immunity lasts.
