I don't know why you think it wouldn't fill in at the 12' height level. It will. Eventually, it will become large such that you will need to keep it sheared in order to keep the size controlled. Look at some Google Images of it and find some that show it with age.

The alternative would be the arborvitae Emerald Green (Thuja occidentalis 'Smaragd') - has the same height as Techny (12-15') but a narrower profile (3') so better for quite narrow screening.

Both of these selections are extremely widely used for privacy screening ('Smaragd' is probably the single most widely sold plant in the entire US!!).

But how long will she be looking at a picket fence of arborvitae? And one has a problem and there's a hole in the fence. Wouldn't a variety of shrubs, staggered, give a fuller screen?

My neighbor and I have used red-tip Photinia here in New Jersey. It's hardy, evergreen, grows quickly, can be pruned as a shrub. Our HOA has allowed the ones in our park to become quite tree-like. Don't know your zone, so check carefully.

Thanks all for the feedback. I think we are going to stick with the Techney. The comment about one needing replacing is a thought I had as well, just worried about spread with trees and shrubs. We are in SW Ohio.