2D Curves are very useful for creating mitered corners in a profile of even thickness. You can construct this arch from just 3 points, and it remains adjustable, by moving them around.
Working in the XY plane, with Transform Pivot set to 'Median'..
- Create a Bezier Curve
- Edit Mode, Right-Click menu, subdivide it once.
- Select the 3 vertices, hit V. set their type to 'Vector'
- Select the left 2 vertices, and SX0 straighten them parallel to Y
- Drag the other vertex roughly where you want the tip of the arch. Doesn't really matter yet.
- In the curve's 'Data' tab > 'Shape' panel, set its type to 2D. In the 'Geometry' panel, give the curve an extrusion to the depth of your arch:
(The blue version is just a rotated copy to show you the extrusion.)
You're almost done.
- (left) Give the curve a Solidify modifier, set to 'Complex' and 'Even'
- (middle) Give the curve a Mirror modifier, across X, with 'Bisect' and possibly 'Flip' in X, too. You'll see..
You can now adjust the shape, depth, and thickness of your arch as much as you like, without reworking joints, etc. At any point, you can convert the curve to a mesh.