First things first. I think your flat top, corner radius thing is ugly. Looks like it's about to collapse. But that's just my personal, untrained eye (along with those of millenia of ancient builders). If it makes you happy, just go with it. You're the one who has to look at it every day.
Personally, I like a smooth, sweeping top curve set between 2 smaller (8 or 9 in.)corner radii. Classically speaking, from the earliest evolution of architecture, the elliptical arch has always represented a symbol of strength and beauty. And it also is a structurally significant shape, with the top compression chord providing strength and stability, while the outward thrust is resisted by strong side columns. With respect to the elliptical shape, I don't have any online references to explain what I'm talking about, but it's all just simple high school geometry. I think if you Google it, something should turn up. Try "elliptical arch with stringline," or variations thereof. In summary, you just anchor each end of a loose string at the ellipse's focal points on the horizontal major axis (centers of the corner radii), leaving enough slack to enable a (held with the hand) pencil to be pushed against the string from the bottom to stretch it tight, while pushing it tightly all the way around, starting at one side's vertical plane, going upwards towards the apogee (highest point, in center) and back down to the opposite vertical plane. It's based on the principle of the equation: ("x" squared divided by "a" squared plus "y" squared divided by "b" squared = 1), meaning the sum of the two straight-line distances between focal points and any place on the periphery is always the same. Trust me, the ancient Greeks/Romans/Egyptians knew what they were doing.
I built a full-sized plywood/fiberglas form (6' span) using the foregoing to construct a brick arch over a wood stove used to heat an entire former house, and it turned out very nice-looking. Realtor said it was the selling point of the place, with everyone who toured it saying they were struck by the arch's beauty and simplicity.