Maximum visual impact is strongly correlated with simplicity. IMO, one of the worst looks is hedging running concurrently, one in front of the other ... a hedge in front of a hedge, and in this case, in front of another hedge (as you describe the grasses and hydrangea. A single hedge would be vastly better. On your own, find local examples of each scheme and see what you think they look like in real life. You don't need to find the exact thing, just a sample of similar.

There's nothing wrong with a long, long row of hydrangeas. I'm not sure I'd stick a row of grasses in front and in back. What I'd recommend is picking a spring-blooming azalea to intersperse with the hydrangeas. (I'm assuming the hydrangeas will bloom in summer.) I'd pick a diversity of heights and put the shorter plant a little in front of the larger. As in Larger in back, next to shorter pulled out a little bit, but not in a different row. Alternate like that.

That will give you a rhythm and diversity. You can choose to do the same plant, but I think a subtle change in color is very nice --- like shades of pink.

Then, later, if you want to deepen the bed, find a set of plants to put in front.

You could also do a massing centered on a smallish tree, where you'd have a smallish azalea or two, then a larger hydrangea (you could reverse the sizing), a smaller shrub or then the tree, then a smaller shrub, then a larger. In short, make a plan with waves of heights.

How high is the retaining wall? If it is for example 6ft high, a boarder planting would be lovely if broken up by a seating area. For example, a border planting 3-5 feet deep against the wall, with an opening for a 5ft long bench set upon flag stones or gravel, with a lattice covered in vine, against the grey wall, behind it. For the planting palette itself, it helps to know the height of the wall and the dimensions of the the area in question. For example, the space is 50ft long x 20ft wide, and the retaining wall is 6ft high.

Thanks everyone. I think I will just go for a row of endless summer hydrangeas.

I would suggest you rethink the Endless Summer idea unless you don't ever get spring frosts or you can cover them all in that event. The problem with Hydrangea macrophylla cultivars, even rebloomers like ES is that once the buds start swelling prior to bloom, they are vulnerable to frost. So if you have a warm February and then a cold dip, you may not get any of the spring blooms at all only the ones that form from new buds later in the year. And I am not sure how well they will do in your area with morning sun, especially with the sun heating up the wall. In my area, northern New England, morning sun is good for any of the Hydrangea macrophylla cultivars (as long as I winter protect) but in TX you may need much more shade based on comments in the Hydrangea forum. There is a poster there from Hurst which is your part of TX, so you might check out his suggestions for morning sun, afternoon shade. I think that any of the Hydrangea paniculata cultivars such as Limelight, Quickfire, Bobo, etc would be better in that spot as far as healthy growth and good blooming.