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First, do not use silicone caulk. It is unpaintable, and will be an eyesore. If you need to form a smooth edge, use a latex paintable caulk. I have used the tape method with a sash brush to finish, and it works just fine. It really does not need to be perfect, most people will not study the texture on the radius, and from a distance the line will look perfect. Kind of like all these pixels on your computer monitor.

I confess, my radius corners that have contrasting paint colors are not perfect. The imperfections are generally 1/8 inch of color intrusion. If that is not tolerable, your caulk method may work, or just use a sanding block to smooth out a narrow line on the radius where you want the paint to join. Then paint over that with the lighest color first, overlapping into the darker color zone. After that has cured, use your masking tape to provide a guide line for the darker color along the center of the sanded radius.