Is that even possible with medial grade wrist devices? Apple Watches can perform it only during sleep which makes sense. It seems like a difficult problem to solve without a chest strap, or just measuring during sleep.

The only other alternative I can think of is a screen strap (some companies make those screenless ones, Polar, Whoop) around the bicep, as it’s relatively close to the shoulder and chest areas which gently move with our breath.

Garmin measures "photoplethysmography-derived respiration" (using the optical HR sensor). Error rates are under 1 breath per minute during sleep or at rest but rises during exercise, up to 4 bpm above the lactate threshold.

Impedance pneumography is more consistently accurate, but requires a chest (not bicep) strap.