If there is a straight part to the arch, I would make a template, on a sled, with hold down, to use on the router table, after cutting close to line on the band saw. You could route, 1 curved end and part of the straight area, then unclamp the piece, and slide to the other end of the template, and route that end.
I made a clamp down sled like that, when making a bunch of boat hand rails. Worked great.
You could just cut very close on the band saw (or right on the line if you can!) and sand to the line on a oss. I prefer to make a template, to get easy, identical perfect radius's.