Sauron's great mistake

[akhtene]

they (numenorians) were already under his power.

I'd say yes, but only due to the encouraged attack on Valinor. I don't believe that even the fallen Numenorians had any grievances against M-E.

Besides, I suppose, time should also be considered. Ar-Pharazon gave ear to Sauron as he wasn't young any more and "was besotted by the fear of death" The qonquest of the whole M-E would have taken years! Could the King afford such time? + the risk of getting killed in a battle? I don't suppose he could agree, even if Sauron suggested that course of action.