Best method I can think of is to use a flap wheel that has been shaped to the approximate radius.
I'm not talking about the cheap little flap wheel sold at hardware stores but industrial duty ones.
I use Norton Resinal Metallite Flap Wheels.
Resin bonded wheels are for steel, glue bonded for aluminium, wood and plastic.
They come in various widths and diameters.
You would like a 1/2" wide by 6" diameter but I think the narrowest they come is 1" .
To shape them I use an old file.
Flap wheels are pretty aggressive but when applied to a sharp corner tend to shred.
I use this to shape the wheel. You have to be sure to have a very good (or braced) grip on the file to prevent it from kicking back.
These wheels tend to wear to a radius when used in corners so redressing should not be required. The radius will not be perfectly constant but will blend sufficiently to look uniform.
Yep Dremel's out.
So is a HD die grinder with ball shaped rotary file.
This will work but is hard to get a uniform surface unless you have a very good eye, are very steady and have a lot of patience.
Other than that all I can think of is milling with a ball end mill.
There are certain finished items on which all the welds should be finished.
Consumer items, works of art and work surfaces on a counter or a bench top among others.
Otherwise I hate to see a weld that has been ground down or reshaped.
This almost always indicates that bad welding is being covered up.
It may also weaken the weld if too much material is removed.