Yes part of me agree's with what you are saying you will always have in the back of your mind what if and is everything ok it doesnt matter how many u/s & tests we have us mums who have lost bubs will always worry.
But you also have to ask yourself what if you get to the 16 weeks with no more u/s or your nuchal scan and you get to 20 weeks and there is something wrong what would you do? would you continue a pregnancy with an serverely abnormal baby if the answer is yes then by all means relax and enjoy your pregnancy.
Myself i have had to end a pregnancy with a serverely disabled child with no prospects of a life and it was terrible i have had every test available with this bub u/s CVS and amnio i had to be sure i could not go through that pain again i would rather m/c early if nature intended then go through what i had to.
But i hope what ever decision you decide makes you relax and feel more optimistic that this pregnancy will be a healthy one for you.