If the poly rope can take a load of 1980 lbs that would give you a safety of approx 1:5.
Thatīs (for me) enough.
But, and theres always one, is that the rope might be difficult to handle, it might not be weather-resistant and so on.
But, test it, and let us know how it works.
Says me, who is thinking in the same direction.....
(Because I canīt get any Amsteel Blue where I live)