What's your question? Ask

English holly should be pruned more lightly, according to the instructions in the following articles:
https://www.rhs.org.uk/plants/details?plantid=6332
https://www.rhs.org.uk/advice/profile?pid=549

The best technique is to remove about 1/3 of the total size of the plant at any one time, so it will take several years to get them down to size. But you can gradually improve their appearance. You can shorten branches and/or remove some branches to thin out the holly.

I agree with your strategy. Make sure to fertilize after the severe pruning. Any idea how the large hole happened? In future, pruning should remove the oldest branches to the ground and not just the growth on the outside. Repeated trimmings of outer growth lead to dense shading of the interior and the dying out of branches. Pruning should let light and air reach the interior which improves overall plant health and aesthetics.

I looked up several references and found a distance of 50 to 300 feet as the maximum spacing to ensure pollination. Your plan to put them 6-8 feet apart is fine. Keep in mind the maximum mature height and spread when spacing your hollies. Six feet may be too close if you have one of the tree varieties.

It is not likely dying, but it does look like sap sucker, woodpecker, or other bird related damage.

You may have to resort to things like bird netting or predator urine to keep them away.

https://www.gardeningknowhow.com/plant-problems/pests/animals/woodpecker-damage-trees.htm

https://www.gardeningknowhow.com/plant-problems/pests/animals/using-predator-urine-in-gardens.htm