I would probably go for cooking the panna cotta in batches. Especially for 150 servings. And the amount of time spent waiting for the cream to come to a boil, would be unbearable for me, too. I tend to hover over pots when I'm boiling bilk/cream, and never leave until they are on a simmer.
Perhaps if you posted your recipe you'd get more input.
I've never made Panna Cotta by boiling anything nor have I ever been concerned about burning the cream. I'm interested to see your approach.
As far as I know, and I am by no means a panna cotta expert, if gelatin is the setting agent, it only needs to be heated enough to dissolve, in fact, if it is boiled, the gelling properties are adversely affected and it will not set.