Block hand sand. I've made quite a few blocks, with different grits and shapes,
for similar sanding.
Some of my blocks are 11-1/4" 2X4s with a 4X24 sanding belt wrapped around it.
Fits my hand comfortably for volume and/or aggressive sanding, when need be.
Some block faces are concave, some convex and some flat for different sanding
I would think your roundovers aren't that great, nor the volume/length, that
hand sanding would take much time or effort. *I'm thinking 1 or 2 item project,
not commercial volume numerous items project.