What's your question? Ask

You will need to spray the stump liberally with a herbicide.
This will move throughout the system and kill the roots.
You will need to then dig up the roots.
Watering the ground around the tree will help loosen the root system.
Work in a circle to pry the root ball from the ground.
After removing the root ball and roots, fill the hole with top soil.

The amount of time it will take to kill the tree will depend on the concentration of the herbicide and the size of the tree.

It can take any where from 1 to 6 weeks according to my research.

Generally, for this application you will want a tight, compact tree that spreads or covers a lot of space. Thujas, compact cedars, and spruces can help here. You will just want something that can be pruned if you are looking for hedgelike structure. This link will give you more information: https://www.gardeningknowhow.com/ornamental/shrubs/shgen/evergreen-shrubs-for-privacy.htm

This could be due to winter damage.

https://www.gardeningknowhow.com/ornamental/trees/arborvitae/winter-damage-to-arborvitae.htm

https://www.gardeningknowhow.com/ornamental/trees/arborvitae/growing-arborvitae-trees.htm

Thuja plicata, western red cedar (the species which is native to the US Pacific Northwest and naturalized in the UK) is a very large growing tree and not appropriate for your small garden.

But there is a smaller hybrid Thuja/arborvitae called "Green Giant" which is popular as hedge plant. This variety may still grow too big for your small garden.

Plant it at least 3 feet from the fence for a 5 year plan, preferably 6 feet or more on a 10 - 20 + year plan, to allow for foliar spread and maintenance access.

https://www.gardeningknowhow.com/ornamental/trees/arborvitae/grow-green-giant-arborvitae.htm

http://www.thujagreengiant.net/how-far-apart-do-you-plant-thuja-green-giant/

Depending on where you are located and availability (not sure what "l4zl0w" means) you may also consider, boxwood, privet, Japanese holly or other; or extend your fence height with a trellis and grow a vine like ivy to achieve the screening you need.

It is, either, one of many Thuja trees, or possibly a Platycladus , but I'm leaning more towards Thuja.

Here is our collection of articles on the Genus Thuja: https://www.gardeningknowhow.com/category/ornamental/trees/arborvitae

We talked, earlier in the year, about how overwatering will take a toll on the trees. This, likely, has to do with the issue that you were having at that time.

Overwatering will cause root rot, paired with the time of planting you can expect a lot of this damage. This is something that needs to be corrected, immediately, though. It will result in the loss of the trees, as it spreads quickly from one to the next.

You will need some heavier fungicides, and to test the soil's pH, and nutrient levels.

A pH meter, and an EC (electrical conductivity) meter will help you here. If you are unfamiliar with how to use them, I would recommend having someone from your local extension come out and test your soil for you.

Likely, they will recommend a fungicide, first.

Here are some articles that will help you to fight what you are seeing here:

https://www.gardeningknowhow.com/extension-search

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

https://www.gardeningknowhow.com/plant-problems/disease/using-lime-sulfur-in-gardens.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm