If you want it to look nice, buy ready-made. If you want a "do it yourself" project, get a vertical milling machine with a tiltable spindle and a round cutter, with a diameter suitable for the finishing radius. If you need custom measurements without major investments, leave it to a carpenter's workshop.
You can't use a 45-degree cutter in a hand router since the radii are different at the outermost and innermost parts of the cutter steel.
A circular saw and jigsaw will look terrible, and after lots of sanding, each post will have four different bevels.
The circular saw and jigsaw didn't give a good result, way too cumbersome. Another option I'm considering is a chisel for the transitions and then a hand plane, followed by a "hand plane with a small nose" to get close to the transition.
It will look a bit strange, the beginning and the end will look slanted.
Possibly if you work from both sides and leave the back intact or grind it down.
The tool for doing it by hand might be one with two handles, the name of which I've forgotten.
