Hi Natalie, thanks for using Ask Extension.

Your Green Giants can be maintained as an 8‑foot formal hedge, but they should not be topped back into bare wood. Arborvitae only regrow reliably from green, leafy tissue, so the safest and most effective method is light, regular shearing once they reach your target height. This lets you hold the height at 8 feet while encouraging the plants to grow together sideways and form a dense, formal wall.
To cap the hedge, wait until the leaders reach the height you want (8 ft). Then trim only the soft green growth at the top—usually just a few inches. Avoid cutting into older brown wood, because arborvitae do not sprout new growth from that area. By removing just the newest green tips, you slow upward growth and redirect energy into lateral thickening. Over time, this creates a tight, uniform top that stays at your chosen height.
As the plants knit together, lightly shear the sides to keep a clean line. Keep the hedge slightly narrower at the top so sunlight reaches the lower branches; this prevents thinning and keeps the screen full, from top to bottom. Plan on one to three light trims per season. Small, frequent cuts are far healthier and more predictable than one heavy cut.
If you prefer the hedge capped at a fixed height through true topping, it’s important to know that arborvitae don’t regrow from old wood. The top may stay flat or have a thin/bare patch permanently. The safest approach is to cut in late winter, make one clean cut across the top at your final height, and then lightly shape the new growth through the season. After topping, the plant will spread sideways rather than upward, which helps it fill in as a hedge, but it won’t regain its natural pointed shape.

Check out the following links for more information:

Maintaining Thuja Green Giant at 15 feet high - Ask Extension

Q&A: Reduce the Height of an Arborvitae? – Maryland Grows

How to rejuvenate a arborvitae - Ask Extension

Thuja 'Green Giant' (Green Giant Arborvitae) | North Carolina Extension Gardener Plant Toolbox

I hope this helps.