If it's Agonis be aware that it is frost tender. Mine (i'm in similar microclimate) freezes to the ground every 3-5 years or is severely 'pruned' back. So not a great choice for a screen in your zone. You might try Leptospermum 'Dark Shadows', which, while also slightly tender, has barely frozen here in the years that I've had both it and the Agonis. Sara

Have you thought about Callistemon (Bottlebrush) - quick growing and make an ideal low maintenance screen. Also, Elaeocarpus reticulatus or eumundii (not sure of the common names of these) I'd go Bottlebrush!

Along with a photo, an indication of where you are is necessary if you are to get useful suggestions since what grows in FL is quite different from NV or here in NH. Here's how to add it to your profile so that it posts automatically by your name when you post so that we don't have to ask. Go to Your Houzz in upper right of every page, click Edit Profile, and on the left side click advanced settings. Well down the advanced settings page is a blank labeled Climate Zone for Garden Forums along with a link to find your zone. If you add info on your state or the nearest large city you will get even better information since zone only relates to average coldest winter temperatures. Then return to the top of the page and click Done Editing.

You want a newly planted tree to provide screening 15 to 20 feet off the ground?!!! No matter how much money you have this is not a realistic goal. The largest tree you could reasonably plant will still take ages to do screening at that height. Like, plan on 10 years. Put a fence on the deck with lattice or whatever and cal it a day. Or put a serious planter with evergreens on the deck.