This is not a good plan. The boards will end up bulging from the pressure of the moist earth if they are attached only at 8' intervals. Figure on tripling the post supports. Much (most) of the 2" x lumber is not treated for ground contact and therefore will not last long as the walls of a planter. If the bottom is open, you don't need extra drainage but the soil in the box, and below the box, should drain well.

Unless it's adorned with some remarkable details or construction methods, I can't conceive of a long, low wooden box spanning all the way across the property as looking good. It sounds like something that most people could not wait to get rid of if they purchased a property with this already on it. I'm pondering why you would do this. If it's just to add another 2' of height to the arborvitaes I'd think finding a taller plant or waiting a little longer would be a better way to go. I do not get it. But if someone held a gun to my head and forced me to design a long lasting raised planter of such dimensions, I'd look into doing it out of some other material first. In Colorado, I'd think acquiring stone would not be as much of a problem as it is in other locales. Honestly, do you really need a raised planter like this?

Basic Unified Construction Standard has the spacing of the posts at 4 feet on center and post size at 4x6 ( I've seen 4x4 used and the wall has held up well so long as your spacing is 4 feet on center and definitely not any higher than 2.5 feet with adequate footing size) .
If the wall is 2.5 ft high the post is set into a concrete footing of 4 ft. deep.
You can run a gravel drainage trench with a silt barrier + a 4" dia. perforated pipe at the bottom of the wall and have it exit the wall and connect to a solid pipe to lead the water out of the box. Maintain a 2% slope of the pipe for best exit flow.

this is a pretty common low cost construction detail seen in the west. It isn't particularly attractive, but it gets the job done.
You might compare the cost of doing a keystone type wall vs. a wood wall. The keystone wall will outlast the wood wall a lot longer and because they do not require any specialized engineering, it may prove to be cost effective for your situation.

We haven't used wood 4x4 in ages, preferring steel I-beams instead when building a wood retaining wall.

Your county might have a set of Uniform Construction Standard details in their building and planning dept.

If we use stone, how would we support where the dirt goes against the back of our fence? This is why we decided on wood, so we could build a large planter box that contains the dirt.