I haven't 3D software for artistic modelling, but elementary methods in a freeware CAD program maybe are applicable also for your purposes. See a screenshot:
In the left there's a sketch drawing, only straight lines on the working plane
In the middle two concentric circles have been added. I have snap to grid ON, so there's no problem to make them concentric and tangential with the line sketch
In the right all unwanted line and curve segments are deleted (=trimmed away) one by one with the eraser.
The next step is to pull some thickness:
The sharp edges can be rounded directly, but as well one can sketch a rounding profile (no need to make it circle, other curves are ok)
Again remove the unwanted arc segment. Then select the sharp corner areas and extrude (=sweep) them along the edge:
After completing the sweep there actually are 3 solid pieces in the same space:
It's fortunately possible to subtract the sweeped solids from the original body. The result = the rounded piece:
As mentioned, sharp edges can be also rounded directly. That obviously is what you called Bevelling:
If the rectangular piece already happens to have the thickness and it doesn't fit to any grid, the case is more difficult:
Fortunately even simplest CAD software have a snapping mode where tangential circles can be drawn. Here a diagonal line has been draw at first through the corners. Its end is the common centerpoint of the circles.
The areas between the circles and the original body can be extruded in CAD programs both to remove and insert the material. Here's a piece is removed by extruding:
After making the rest of the needed extrusions and removing the unwanted splinters, we have the solid arch:
BTW: Direct rounding works in this CAD program also with sharp corners:
The used program = DesignSpark Mechanical, it's a heavily disabled version of SpaceClaim Engineer, but still a valuable tool.