I wonder if you've heard of the Resonance Sonomanometry method claimed by Esperto Medical https://esperto.health/ and how you think this compares to others.

It seems like it might be an ultrasonic method? They claim no calibration needed, but I don't know if it is tolerant to movement and posture.

I can also imagine this might be too power intensive to put in a smartwatch form factor. Maybe more of a bedside or clinical device...

Disclaimer: I had past contact with some of their researchers in a different bioimaging domain. I don't know anything more about this venture than it says on the website.