I have 12 Green Giants planted on my half acre property to screen neighbors' sheds - 6 in a row at two different places. I am in Northern VA zone 7a - not very different from you in NJ. The 12 trees were 12 ft tall when planted and were placed about 6 ft apart 10 years ago to create a screening hedge. They are now about 20 ft tall. They are beautiful and highly functional as a screen and backdrop. We've had no problem with disease - as you say, pretty bulletproof. Only one slight problem: During heavy winter snows they have been completely bent over due to the weight of wet snow - we go out and shake off what we can. In the spring they remained bent for a month or so but by the summer they had righted themselves. (I think one year after such an event, my husband tied rope around the trunk and branches to help them right themselves.) Every year they get wider. Now they are probably at 8-10 ft. wide. I have had to move what I planted in front of them because the trees have consumed that space - so learn from my example and give them as much space as you can. If one was planted by itself I believe it would surely grow 20 ft wide eventually - I have seen them in fact at about 40 ft tall and 15 ft wide. But in hedge conditions they stay more compact. Some that were planted under the neighbor's high trees stopped growing when they reached close to the boughs of the deciduous trees overhead - at about 15 ft. The neighbor has since removed their tree so the arborvitae trees are now growing again, catching up to their siblings. They like full sun and the back sides that don't get full sun are not fully greened out (but we don't see that side so it doesn't bother us - or the tree.) They would be very easy to take down safely if it were ever needed (heaven forbid!) - the boughs can easily be removed then the trunk cut in chunks. Let me know if you want to see pictures and I can take some and post them. Best wishes.
Q
Informative Green Giant post on size of tree planted 10 years ago.
Q