Now the image is clean, we can safely sharpen. (Sharpen before denoise and you'd just amplify grain, which is why it waits until here.)
The unsharp mask is the classic trick: blur a copy, subtract it from the original to isolate the edges, then add those edges back amplified. More amount = more bite. But there's a hard ceiling, past a point the mask stops adding detail and starts tracing halos: bright and dark rings drawn around every high-contrast edge.
During tuning we compared 0.85 (too soft), 0.95/1.3
(crisp and clean) and 1.0/1.5 (halos, dark/blue rings around every
high-contrast edge). CAS (Contrast-Adaptive Sharpening) does the
gentle, halo-free lifting; the unsharp mask, run luma-only so it
adds no colored fringes, supplies the extra bite. 0.95/1.3 is the
honest maximum for this footage.
strength: 0.95 (CAS),
unsharp: 1.3 (luma-only). Visibly sharper, no ringing.