Arborvitae Woodwardii (?) questions

Hi folks,

I have two (I believe) arborvitae woodwardii in my front yard. They are now approaching the height of the front window - they are currently 5.5 feet tall - and I am wondering if I should be trimming them to control the height. I have seen online that these have a mature height of 5-6 feet. If they max out at 6 feet then I would be fine with it, but if they're going to continue growing to 8 feet then I would like to control their height so they don't start blocking the front window.

Photos here: https://imgur.com/a/b1Y35bl

My questions are:
- Is there somehow to verify their "mature" height?
- What happens when they reach their mature height? Do they simply stop growing?
- Should I be pruning them if I want to ensure they do not start covering the front window?
- How/when should I be pruning them?
- I have noticed some sections have the foliage turning black. See the final photo. What is happening?

I am in Alberta, Canada, and these are south-facing.

Thanks folks!