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?

The larger the planter boxes the better, but you can get by with smaller dimensions but the plant will get rootbound quicker. I would recommend a clumper, as it will do much better in a planter. If you choose a variety that is too 'thick' or grows too tightly together, you can simply take out some of the culms to give a more open appearance. I don't know of any small(12 foot) varieties that grow completely upright. You can top culms to any height you like and they will remain at that height, that is, if you choose a larger variety. What is you lowest temps in Winter? There are a number of Bambusas that will do well, although they lean out a bit. Here are some Bambusas to choose from...ABS B. glaucophylla aka MALAY DWARF is a beauty if you like variegated leaves. It does grow as more of a bush and isn't very cold hardy. Others, are some of the B. multiplexes. Kt

Asking a contractor to match concrete is akin to calling in a baker and asking them to match the wedding cake, that you happen to have a sample of in your freezer from a couple of years ago. While they can get it close enough that everyone will agree that it's white and cake, there are going to be differences that the more discerning palates will undoubtedly notice. In your last picture, it looks to me like the contractor used a coarse broom finish, which is an extremely common finish. How rough it is depends a lot on the broom and the broom operator. There is no way to grind/"sand" this smooth without changing the character to yet another version of concrete that doesn't match your steps. If your contractor thinks there is, I suggest you'll be in for another surprise. (You had best get a sample of the end result before committing to the change.) Myself, I much prefer a ground finish over a broom finish. It's smooth and starts looking like low contrast terrazzo. (BTW, do not use an aggressive diamond blade for grinding or you'll be grinding it a second time in order to remove deep grooves. Grinding with carborundum stones would be all you need for smoothing. Also, keep in mind that it's not a 1-hour job. It would be more like a day of grinding ... so would cost extra.)

Seal the inside (and do the outside as well) of your planter boxes. There are various, natural based, plant-friendly waterproof sealers on the market made for this purpose.